GEM-STONES
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t'LATK I
'rftttisfiece
-c
V/L'AMAKISE
I. SAI-I'HIRE 12. YELLOW SAI'FHIli
(Oriental Tofaz)
GEM-STONES
GEM-STONES
AND THEIR DISTINCTIVE CHARACTERS
G. F. HERBERT SMITH
M.A., D.Sc.
OF THE BRITISH MUSEUM (NATURAL HISTORY)
WITH MANY DIAGRAMS AND THIRTY-TWO PLATES
OF WHICH THREE ARE IN COLOUR
THIRD EDITION
METHUEN & CO. LTD.
36 ESSEX STREET W.G.
LONDON
First Published . . . March lit
Second Edition . . . June
Third Editum .
PREFACE
IN this edition the opportunity has been taken to
correct a few misprints and mistakes that have
been discovered in the first, and to alter slightly one
or two paragraphs, but otherwise no change has been
made. G. F. H. S.
WANDSWORTH COMMON, S.W.
PREFACE TO THE FIRST
EDITION
IT has been my endeavour to provide in this book
a concise, yet sufficiently complete, account of
the physical characters of the mineral species which
find service in jewellery, and of the methods available
for determining their principal physical constants to
enable a reader, even if previously unacquainted with
the subject, to have at hand all the information
requisite for the sure identification of any cut stone
which may be met with. For several reasons I have
dealt somewhat more fully with the branches of
science closely connected with the properties of
crystallized matter than has been customary hitherto
in even the most comprehensive books on precious
2005117
vi GEM-STONES
stones. Recent years have witnessed many changes
in the jewellery world. Gem-stones are no longer
entirely drawn from a few well-marked mineral
species, which are, on the whole, easily distinguishable
from one another, and it becomes increasingly diffi-
cult for even the most experienced eye to recognize
a cut stone with unerring certainty. So long as the
only confusion lay between precious stones and paste
imitations an ordinary file was the solitary piece of
apparatus required by the jeweller, but now recourse
must be had to more discriminative tests, such as
the refractive index or the specific gravity, the de-
termination of which calls for a little knowledge and
skill. Concurrently, a keener interest is being taken
in the scientific aspect of gem-stones by the public
at large, who are attracted to them mainly by
aesthetic considerations.
While the treatment has been kept as simple as
possible, technical expressions, where necessary, have
not been avoided, but their meanings have been
explained, and it is hoped that their use will not
prove stumbling-blocks to the novice. Unfamiliar
words of this kind often give a forbidding air to a
new subject, but they are used merely to avoid cir-
cumlocution, and not, like the incantations of a
wizard, to veil the difficulties in still deeper gloom.
For actual practical work the pages on the refracto-
meter and its use and the method of heavy liquids
for the determination of specific gravities, and the
tables of physical constants at the end of the book,
with occasional reference, in case of doubt, to the
descriptions of the several species alone are required ;
other methods — such as the prismatic mode of
measuring refractive indices, or the hydrostatic way
PREFACE vii
of finding specific gravities — which find a place in
the ordinary curriculum of a physics course are
described in their special application to gem-stones,
but they are not so suitable for workshop practice.
Since the scope of the book is confined mainly to
the stones as they appear on the market, little has
been said about their geological occurrence ; the case
of diamond, however, is of exceptional interest and
has been more fully treated. The weights stated
for the historical diamonds are those usually pub-
lished, and are probably in many instances far from
correct, but they serve to give an idea of the sizes of
the stones ; the English carat is the unit used, and
the numbers must be increased by about 2\ per cent,
if the weights be expressed in metric carats. The
prices quoted for the various species must only be
regarded as approximate, since they may change
from year to year, or even day to day, according to
the state of trade and the whim of fashion.
The diagram on Plate II and most of the crystal
drawings were made by me. The remaining draw-
ings are the work of Mr. H. H. Penton. He likewise
prepared the coloured drawings of cut stones which
appear on the three coloured plates, his models, with
two exceptions, being selected from the cut specimens
.in the Mineral Collection of the British Museum by
permission of the Trustees. Unfortunately, the
difficulties that still beset the reproduction of pictures
in colour have prevented full justice being done
to the faithfulness of his brush. I highly appreciate
the interest he took in the work, and the care and
skill with which it was executed. My thanks are
due to the De Beers Consolidated Mines Co. Ltd.,
and to Sir Henry A. Miers, F.R.S., Principal of the
viii GEM-STONES
University of London, for the illustrations of the
Kimberley and Wesselton diamond mines, and of the
methods and apparatus employed in breaking up
and concentrating the blue ground ; to Messrs. I. J.
Asscher & Co. for the use of the photograph of the
Cullinan diamond ; to Mr. J. H. Steward for the loan
of the block of the refractometer ; and to Mr. H. W.
Atkinson for the illustration of the diamond-sorting
machine. My colleague, Mr. W. Campbell Smith,
B.A., has most kindly read the proof-sheets, and has
been of great assistance in many ways. I hope that,
thanks to his invaluable help, the errors in the book
which may have escaped notice will prove few in
number and unimportant in character. To Mr.
Edward Hopkins I owe an especial debt of gratitude
for his cheerful readiness to assist me in any way
in his power. He read both the manuscript and the
proof-sheets, and the information with regard to the
commercial and practical side of the subject was
very largely supplied by him. He also placed at
my service a large number of photographs, some of
which — for instance, those illustrating the cutting of
stones — he had specially taken for me, and he pro-
cured for me the jewellery designs shown on Plates
IV and V.
If this book be found by those engaged in the
jewellery trade helpful in their everyday work, and
if it wakens in readers generally an appreciation of
the variety of beautiful minerals suitable for gems,
and an interest in the wondrous qualities of crystal-
lized substances, I shall be more than satisfied.
G. F. H. S.
WANDSWORTH COMMON, S.W.
CONTENTS
CHAP. PACK
I. INTRODUCTION ..... i
PART I— SECTION A
THE CHARACTERS OF GEM-STONES
II. CRYSTALLINE FORM . . . . .6
III. REFLECTION, REFRACTION, AND DISPERSION . 14
IV. MEASUREMENT OF REFRACTIVE INDICES. . 21
V. LUSTRE AND SHEEN . . . .37
VI. DOUBLE REFRACTION . . . .40
VII. ABSORPTION EFFECTS : COLOUR, DICHROISM,
ETC 53
VIII. SPECIFIC GRAVITY . . . .63
IX. HARDNESS AND CLEAVABILITY . . .78
X. ELECTRICAL CHARACTERS . . . .82
PART I— SECTION B
THE TECHNOLOGY OF GEM-STONES
XI. UNIT OF WEIGHT . . . . .84
XII. FASHIONING OF GEM-STONES . . .88
XIII. NOMENCLATURE OF PRECIOUS STONES . . 109
XIV. MANUFACTURED STONES . . . .113
XV. IMITATION STONES . . . . .124
ix
GEM-STONES
PART II— SECTION A
PRECIOUS STONES
CHAP.
XVI. DIAMOND.
XVII. OCCURRENCE OF DIAMOND . . • 137
XVIII. HISTORICAL DIAMONDS. . , • 157
XIX. CORUNDUM (Sapphire, Ruby) . . .172
XX. BERYL (Emerald, Aquamarine, Morganite) . 184
PART II— SECTION B
SEMI-PRECIOUS STONES
XXI. TOPAZ 197
XXII. SPINEL (Balas-Ruby, Rubicelle) . . 203
XXIII. GARNET 207
(a) HESSONITE (Grossular, Cinnamon-Stone,
Hyacinth, Jacinth} . . .211
(b) PYROPE (' Cape- Ruby'} . . .212
(<r) RHODOLITE . . . .214
(d) ALMANDINE (Carbuncle) . . .214
(e) SPESSARTITE . . . .216
(/) ANDRADITE (Demantoid, Topazolite,
'Olivine'). . . . .216
(g) UVAROVITE . . . .218
XXIV. TOURMALINE (Rubellite) . . .219
XXV. PERIDOT . . . . . .225
XXVI. ZIRCON (Jargoon, Hyacinth, Jacinth] . . 228
XXVII. CHRYSOBERYL (Chrysolite, Cats-Eye, Cymo-
phane, Alexandrite) .... 233
XXVIII. QUARTZ (Rock- Crystal, Amethyst, Citrine,
Cairngorm, Cafs-Eye, Tigers-Eye) . . 238
XXIX. CHALCEDONY, AGATE, ETC. . . . 246
CONTENTS xi
XXX. OPAL (White Opal, Black Opal, Fire-Opal) 249
XXXI. FELSPAR (Moonstone, Sunstone, Labra-
dorite, Amazon-Stone) . . -254
XXXII. TURQUOISE, ODONTOLITE, VARISCITE . 257
XXXIII. JADE (Nephrite or Greenstone, Jadeite) . 260
XXXIV. SPODUMENE (Kunzite, Hiddenite], IOLITE,
BENITOITE . . . . .265
XXXV. EUCLASE, PHENAKITE, BERYLLONITE . 269
XXXVI. ENSTATITE ('Green Garnet'}, DIOPSIDE,
KYANITE, ANDALUSITE, IDOCRASE, EPI-
DOTE, SPHENE, AXINITE, PREHNITE,
APATITE, DIOPTASE . . .271
XXXVII. CASSITERITE, ANATASE, PYRITES, HEMATITE 281
XXXVIII. OBSIDIAN, MOLDAVITE . . .283
PART II— SECTION C
ORNAMENTAL STONES
XXXIX. FLUOR, LAPIS LAZULI, SODALITE, VIOLANE,
RHODONITE, AZURITE, MALACHITE,
THULITE, MARBLE, APOPHYLLITE,
CHRYSOCOLLA, STEATITE OR SOAPSTONE,
MEERSCHAUM, SERPENTINE . . 285
PART II— SECTION D
ORGANIC PRODUCTS
XL. PEARL, CORAL, AMBER . . .291
TABLES
I. CHEMICAL COMPOSITION OF GEM-STONES . 300
II. COLOUR OF GEM-STONES . . .301
III. REFRACTIVE INDICES OF GEM-STONES . 302
GEM-STONES
PAG!
IV. COLOUR-DISPERSION OF GEM-STONES . . 303
V. CHARACTER OF THE REFRACTION OF GEM-
STONES . . . . . -303
VI. DICHROISM OF GEM-STONES . . . 304
VII. SPECIFIC GRAVITIES OF GEM-STONES . . 305
VIII. DEGREES OF HARDNESS OF GEM-STONES . 305
IX. DATA . . . . . . .306
INDEX ...... 307
LIST OF PLATES
PAGE
I. GEM-STONES (in colour) . . . Frontispiece
II. REFRACTIVE INDEX DIAGRAM. . . 36
III. INTERFERENCE FIGURES . . .48
IV. JEWELLERY DESIGNS . . . . 62
V. JEWELLERY DESIGNS . . . .88
VI. APPLIANCES USED FOR POLISHING DIAMONDS 102
VII. POLISHING DIAMONDS . . . .103
VIII. SLITTING AND POLISHING COLOURED STONES 104
IX. FACETING MACHINE . . . .105
X. LAPIDARY'S WORKSHOP AND OFFICE IN
ENGLAND . . . . 106
XL LAPIDARY'S WORKSHOP IN RUSSIA . . 107
XII. FRENCH FAMILY CUTTING STONES . . 108
XIII. INDIAN LAPIDARY . . . .109
XIV. BLOWPIPE USED FOR THE MANUFACTURE
OF RUBIES AND SAPPHIRES . . .118
XV. KlMBERLEY MlNE, 1 87 1 . . . .140
XVI. KlMBERLEY MINE, 1872. . . .141
XVII. KlMBERLEY MlNE, 1874. . . .142
XVIII. KlMBERLEY MlNE, l88l . . .143
XIX. KlMBERLEY MlNE AT THE PRESENT DAY . 144
XX. WESSELTON (open) MINE . . .145
XXI. LOADING THE BLUE GROUND ON THE
FLOORS, AND PLOUGHING IT OVER . . 146
XXII. WASHING-MACHINES FOR CONCENTRATING
THE BLUE GROUND . . . .147
XXIII. DIAMOND-SORTING MACHINES. . . 148
xiii
xiv GEM-STONES
PAGE
XXIV. KAFFIRS PICKING OUT DIAMONDS . . 149
XXV. CULLINAN DIAMOND (natural size) . . 168
XXVI. LARGE AQUAMARINE CRYSTAL (one-sixth
natural size), FOUND AT MARAMBAYA,
MINAS GERAES, BRAZIL . . . .196
XXVII. GEM-STONES (in colour) . . .226
XXVIII. OPAL MINES, WHITE CLIFFS, NEW SOUTH
WALES ...... 252
XXIX. GEM-STONES (in colour) . . .256
XXX. NATIVES DRILLING PEARLS . . . 294
XXXI. METAL FIGURES OF BUDDHA INSERTED IN A
PEARL-OYSTER . . . . 296
XXXII. SECTIONS OF CULTURE PEARL . . 297
GEM-STONES
GEM-STONES
CHAPTER 1
INTRODUCTION
BEAUTY, durability, and rarity: such are the
three cardinal virtues of a perfect gem-stone.
Stones lacking any of them cannot aspire to a high
place in the ranks of precious stones, although it
does not necessarily follow that they are of no use
for ornamental purposes. The case of pearl, which,
though not properly included among gem-stones,
being directly produced by living agency, yet holds
an honoured place in jewellery, constitutes to some
extent an exception, since its incontestable beauty
atones for its comparative want of durability.
That a gem-stone should be a delight to the eye
is a truism that need not be laboured ; for such is
its whole raison d'etre. The members of the Mineral
Kingdom that find service in jewellery may be
divided into three groups, according as they are
transparent, translucent, or opaque. Of these the
first, which is by far the largest and the most
important, may itself be further sub-divided into
two sections: stones which are devoid of colour,
and stones which are tinted. Among the former,
diamond reigns supreme, since it alone possesses
2 GEM-STONES
that marvellous ' fire,' oscillating with every move-
ment from heavenly blue to glowing red, which is
so highly esteemed and so much besought. Other
stones, such as ' fired ' zircon, white sapphire, white
topaz, and rock-crystal, may dazzle with brilliancy
of light reflected from the surface or emitted from
the interior, but none of them, like diamond, glow
with mysterious gleams. No hint of colour, save
perhaps a trace of the blue of steel, can be tolerated
in stones of this category ; above all is a touch of the
jaundice hue of yellow abhorred. It taxes all the skill
of the lapidary to assure that the disposition of the
facets be such as to reveal the full splendour of the
stone. A coloured stone, on the other hand, depends
for its attractiveness more upon its intrinsic hue
than upon the manner of its cutting. The tint must
not be too light or too dark in shade : a stone that
has barely any colour has little interest, and one
which is too dark appears almost opaque and
black. The lapidary can to some extent remedy
these defects by cutting the former deep and the
latter shallow. In certain curious stones — for
instance tourmaline — the transparency, and in others
— such as ruby, sapphire, and one of the recent
additions to the gem world, kunzite — the colour,
varies considerably in different directions. The
colours that are most admired — the fiery red of
ruby, the royal blue of sapphire, the verdant green
of emerald, and the golden yellow of topaz — are
pure tints, and the absorption spectra corresponding
to them are on the whole continuous and often
restricted. They therefore retain the purity of
their colour even in artificial light, though certain
sapphires transmit a relatively larger amount of red,
INTRODUCTION 3
and consequently turn purple at night. Of the
small group of translucent stones which pass light,
but are not clear enough to be seen through, the
most important is opal. It and certain others
of the group owe their merit to the same optical
effect as that characterizing soap-bubbles, tarnished
steel, and so forth, and not to any intrinsic coloration.
Another set of stones — moonstone and the star-
stones — reflect light from the interior more or less
regularly, but not in such a way as to produce a
play of colour. The last group, which comprises
opaque stones, has a single representative among
ordinary gem-stones, namely, turquoise. In this
case light is scattered and reflected from layers
immediately contiguous to the surface, and the
colour is due to the resulting absorption. The
apparent darkness of a deep-coloured stone follows
from a different cause : the light passing into the
stone is wholly absorbed within it, and, since none
is emitted, the stone appears black. The claims of
turquoise are maintained by the blue variety ; there
is little demand for stones of a greenish tinge.
It is evidently desirable that any stones used in
jewellery should be able to resist the mechanical
and chemical actions of everyday life. No one is
anxious to replace jewels every few years, and the
most valuable stones are expected to endure for
all time. The mechanical abrasion is caused by
the minute grains of sand that are contained in
ordinary dust, and gem-stones should be at least
as hard as they — a condition fulfilled by all the
principal species with the exception of opal, turquoise,
peridot, and demantoid. Since the beauty of the
first named does not depend on the brilliancy of its
4 GEM-STONES
polish, scratches on the surface are not of much
importance ; further, all four are only slightly softer
than sand. It may be noted that the softness of paste
stones, apart from any objections that may be felt
to the use of imitations, renders them unsuitable for
jewellery purposes. The only stones that are likely
to be chemically affected in the course of wear are
those which are in the slightest degree porous. It
is hazardous to immerse turquoises in liquids, even in
water, lest the bluish green colour be oxidized to the
despised yellowish hue. The risk of damage to opals,
moonstones, and star-stones by the penetration of
dirt or grease into the interior of the stones is less,
but is not wholly negligible. Similar remarks apply
with even greater force to pearls. Their charm, which
is due to a peculiar surface-play of light, might be
destroyed by contamination with grease, ink, or similar
matter ; they are, moreover, soft. For both reasons
their use in rings is much to be deprecated. Nothing
can be more unsightly than the dingy appearance of
a pearl ring after a few years' wear.
It cannot be gainsaid that mankind prefers the
rare to the beautiful, and what is within reach of
all is lightly esteemed. It is for this reason that
garnet and moonstone lie under a cloud. Purchasers
can readily be found for a ' Cape-ruby ' or an
'olivine,' but not for a garnet; garnets are so
common, is the usual remark. Nevertheless, the
stones mentioned are really garnets. If science
succeeded in manufacturing diamonds at the cost
of shillings instead of the pounds that are now asked
for Nature's products — not that such a prospect is
at all probable or even feasible — we might expect
them to vanish entirely from fashionable jewellery.
INTRODUCTION 5
A careful study of the showcases of the most
extensive jewellery establishment brings to light the
fact that, despite the apparent profusion, the number
of different species represented is restricted.
Diamond, ruby, emerald, sapphire, pearl, opal,
turquoise, topaz, amethyst are all that are ordinarily
asked for. Yet, as later pages will show, there are
many others worthy of consideration ; two among
them — peridot and tourmaline — are, indeed, slowly
becoming known. For the first five of the stones
mentioned above, the demand is relatively steady,
and varies absolutely only with the purchasing
power of the world ; but a lesser known stone may
suddenly spring into prominence owing to the caprice
of fashion or the preference of some great lady or
leader of fashion. Not many years ago, for instance,
violet was the favourite colour for ladies' dresses,
and consequently amethysts were much worn to
match, but with the change of fashion they speedily
sank to their former obscurity. Another stone may
perhaps figure at some royal wedding; for a brief
while it becomes the vogue, and afterwards is
seldom seen.
Except that diamond, ruby, emerald, and
sapphire, and, we should add, pearl, may indis-
putably be considered to occupy the first rank, it
is impossible to form the gem-stones in any strict
order. Every generation sees some change. The
value of a stone is after all merely what it will
fetch in the open market, and its artistic merits may
be a matter of opinion. The familiar aphorism,
de gustibus non est disputandum, is a warning not
to enlarge upon this point.
PART I— SECTION A
THE CHARACTERS OF GEM-
STONES
CHAPTER II
CRYSTALLINE FORM
WITH the single exception of opal, the whole
of the principal mineral species used in
jewellery are distinguished from glass and similar
substances by one fundamental difference : they are
crystallized matter, and the atoms composing them
are regularly arranged throughout the structure.
The words crystal and glass are employed in
science in senses differing considerably from those
in popular use. The former of them is derived
from the Greek word «pvo?, meaning ice, and was
at one time used in that sense. For instance, the
old fourteenth-century reading of Psalm cxlvii. 1 7,
which appears in the authorized version as " He
giveth his ice like morsels," ran " He sendis his
kristall as morcels." It was also applied to the
beautiful, lustrous quartz found among the eternal
snows of the Alps, since, on account of their
limpidity, these stones were supposed, as Pliny tells
us, to consist of water congealed by the extreme
CRYSTALLINE FORM 7
cold of those regions ; such at the present day is
the ordinary meaning of the word. But, when
early investigators discovered that a salt solution
on evaporation left behind groups of slender
glistening prisms, each very similar to the rest, they
naturally — though, as we now know, wrongly —
regarded them as representing yet another form
of congealed water, and applied the same word to
such substances. Subsequent research has shown
that these salts, as well as mineral substances
occurring with natural faces in nature, have in
common the fundamental property of regularity of
arrangement of the constituent atoms, and science
therefore defines by the word crystal a substance
in which the structure is uniform throughout, and
all the similar atoms composing it are arranged with
regard to the structure in a similar way.
The other word is yet more familiar ; it denotes
the transparent, lustrous, hard, and brittle substance
produced by the fusion of sand with soda or potash
or both which fills our windows and serves a variety
of useful purposes. Research has shown that
glass, though apparently so uniform in character,
has in reality no regularity of molecular arrange-
ment. It is, in fact, a kind of mosaic of atoms,
huddled together anyhow, but so irregular is its
irregularity that it simulates perfect regularity.
Science uses the word glass in this widened mean-
ing. Two substances may, as a matter of fact,
have the same chemical composition, and one be
a crystal and the other a glass. For example,
quartz, if heated to a high temperature, may be
fused and converted into a glass. The difference
in the two types of structure may be illustrated
8 GEM-STONES
by a comparison between a regiment of soldiers
drawn up on parade and an ordinary crowd of
people.
The crystalline form is a visible sign of the
molecular arrangement, and is intimately associated
with the directional physical properties, such as the
optical characters, cleavage, etc. A study of it is
not only of interest in itself, but also of great
importance to the lapidary who wishes to cut a stone
to the best advantage, and it is, moreover, of service
in distinguishing stones when in the rough state.
The development of natural faces on a crystal
FIG. I.— Cubo-Octahedra.
is far from being haphazard, but is governed by
the condition that the angles between similar faces,
whether on the same crystal or on different crystals,
are equal, however varying may be the shapes and
the relative sizes of the faces (Fig. i), and by the
tendency of the faces bounding the crystal to fall
into series with parallel edges, such series being
termed zones. Each species has a characteristic
type of crystallization, which may be referred to
one of the following six systems : —
I. Cubic. — Crystals in this system can be re-
ferred to three edges, which are mutually at right
angles, and in which the directional characters are
identical in value. These principal edges are known
CRYSTALLINE FORM
as axes. Some typical forms are the cube (Fig. 2),
characteristic of fluor ; the octahedron (Fig. 3),
characteristic of diamond and spinel ; the dodeca-
hedron (Fig. 4), characteristic of garnet; and the
FIG.
Cube. FIG. 3.— Octahedron. FIG. 4.— Dodecahedron.
triakisoctahedron, or three-faced octahedron (Fig. 5).
All crystals belonging to this system are singly
refractive.
2. Tetragonal. — Such crystals can be referred
FIG. 5. — Triakis-
octahedron, or
Three-faced Oc-
tahedron.
FIG. b. — Tetra-
gonal Crystal.
to three axes, which are mutually at right angles,
but in only two of them are the directional characters
identical. A typical form is a four-sided prism,
mm, of square section, terminated by four triangular
faces,/* (Fig. 6), the usual shape of crystals of zircon
and idocrase.
10
GEM-STONES
Crystals belonging to this system are doubly
refractive and uniaxial, i.e. they have one direction
of single refraction (cf. p. 45), which is parallel to
the unequal edge of the three mentioned above.
H 3. Hexagonal.- — Such crystals
can be referred alternatively
either to a set of three axes,
X, Y, Z (Fig. 7), which lie in
a plane perpendicular to a fourth,
H, and are mutually inclined at
angles of 60°, or to a set of
three, a, b, c, which are not at
FIG. 7.-T wo alternative right angles as in the cubic
system, but in which the direc-
tional characters are identical.
sets of Axes in the
Hexagonal System.
The fourth axis in the first arrangement is equally
inclined to each in the second set of axes. Many
important species crystallize in
this system — corundum (sapphire,
ruby), beryl (emerald, aqua-
marine), tourmaline, quartz, and
phenakite. The crystals usually
FIGS. 8-10. — Hexagonal Crystals.
display a six-sided prism, terminated by a single
face, c, perpendicular to the edge of the prism m
(Fig. 8), e.g. emerald, or by six or twelve inclined
faces, p (Fig. 9), e.g. quartz, crystals of which are
CRYSTALLINE FORM
1 1
so constant in form as to be the most familiar in the
Mineral Kingdom. Tourmaline crystals (Fig. 10)
are peculiar because of the fact that often one end
is obviously to the eye flatter than the other.
Crystals belonging to this system are also doubly
refractive and uniaxial, the direction of single
refraction being parallel to the fourth axis mentioned
above, and therefore also parallel to the prism edge.
Hence deeply coloured tourmaline, which strongly
absorbs the ordinary ray, must be cut with the
table-facet parallel to the edge
of the prism.
4. Orthorhombic. — Such
crystals can be referred to
three axes, which are mutu-
ally at right angles, but in
which each of the directional
characters are different. The
crystals are usually prismatic FlG> Vi. -Relation of the
in shape, One of the axes two directions of single
being parallel to the prism Refraction to the Axes
, _ • 1 . j in an Orthorhombic
edge. Topaz, peridot, and Crystal
chrysoberyl are the most
important species crystallizing in this system.
Crystals belonging to this system are doubly
refractive and biaxial, i.e. they have two directions
of single refraction (cf. p. 45). The three axes
a, d, c (Fig. 1 1) are parallel respectively to the two
bisectrices of the directions of single refraction,
and the direction perpendicular to the plane con-
taining those directions.
5. Monoclinic. — Such crystals can be referred to
three axes, one of which is at right angles to the
other two, which are, however, themselves not at
12 GEM-STONES
right angles. Spodumene (kunzite) and some
moonstone crystallize in this system.
Crystals belonging to this system are doubly
refractive and biaxial, but in this case the first axis
alone is parallel to one of the principal optical
directions.
6. Tridinic. — Such crystals have no edges at
right angles, and the optical characters are not
immediately related to the crystalline form. Some
moonstone crystallizes in this system.
Crystals are often not single separate individuals.
For instance, diamond and spinel are found in fiat
triangular crystals with their girdles
cleft at the corners (Fig. 1 2). Each
of such crystals is really composed
of portions of two similar octahedra,
which are placed together in such
a way that each is a reflection of
FIG. i2.— Twinned the other. Such composite crystals
Octahedron. are called twins or macles. Some-
times the twinning is repeated, and
the individuals may be so minute as to call for a
microscope for their perception.
A composite structure may also result from the
conjunction of numberless minute individuals
without any definite orientation, as in the case of
chalcedony and agate. So by supposing the
individuals to become infinitesimally small, we pass
to a glass-like substance.
It is often a peculiarity of crystals of a species
to display a typical combination of natural faces.
Such a combination is known as the habit of the
species, and is often of service for the purpose of
identifying stones before they are cut. Thus, a
CRYSTALLINE FORM 13
habit of diamond and spinel is an octahedron, often
twinned, of garnet a dodecahedron, of emerald a
flat-ended hexagonal prism, and so on.
It is one of the most interesting and remarkable
features connected with crystallization that the
composition and the physical characters — for instance,
the refractive indices and specific gravity — may,
without any serious disturbance of the molecular
arrangement, vary considerably owing to the more
or less complete replacement of one element by
another closely allied to it. That is the cause of
the range of the physical characters which has been
observed in such species as tourmaline, peridot,
spinel, etc. The principal replacements in the
case of the gem-stones are ferric oxide, Fe2O3, by
alumina, A12O3, and ferrous oxide, FeO, by magnesia,
MgO.
A list of the principal gem-stones, arranged by
their chemical composition, is given in Table I at
the end of the book.
CHAPTER III
REFLECTION, REFRACTION, AND DISPERSION
IT is obvious that, since a stone suitable for
ornamental use must appeal to the eye, its
most important characters are those which depend
upon light ; indeed, the whole art of the lapidary
consists in shaping it in such a way as to show
these qualities to the best advantage. To under-
stand why certain forms are given to a cut stone,
it is essential for us to ascertain what becomes of
the light which falls upon the surface of the stone ;
further, we shall find that the action of a stone upon
light is of very great help in distinguishing the
different species of gem-stones. The phenomena
displayed by light which impinges upon the surface
separating two media1 are very similar in character,
whatever be the nature of the media.
Ordinary experience with a plane mirror tells us
that, when light is returned, or reflected, as it is
usually termed, from a plane or flat surface, there
is no alteration in the size of objects viewed in this
way, but that the right and the left hands are inter-
changed : our right hand becomes the left hand in
1 The word medium is employed by physicists to express any sub-
stance through which light passes, and includes solids such as glass,
liquids such as water, and gases such as air ; the nature of the substance
is not postulated.
REFLECTION, REFRACTION, DISPERSION 15
our reflection in the mirror. We notice, further, that
our reflection is apparently just as far distant from
the mirror on the farther side as we are on this
side. In Fig. 1 3 MM' is a section of the mirror,
and O' is the image of the hand O as seen in the
mirror. Light from O reaches the eye E by way
of m, but it appears to come from O. Since OO
is perpendicular to the mirror, and O and O lie at
equal distances from it, it follows from elementary
FIG. 13.— Reflection at a Plane Mirror.
geometry that the angle z", which the reflected ray
makes with win, the normal to the mirror, is equal
to 2, the angle which the incident ray makes with
the same direction.
Again, everyday experience tells us that the
case is less simple when light actually crosses the
bounding surface and passes into the other medium.
Thus, if we look down into a bath filled with water,
the bottom of the bath appears to have been raised
up, and a stick plunged into the water seems to be
1 6 GEM-STONES
bent just at the surface, except in the particular
case when it is perfectly upright. Since the stick
itself has not been bent, light evidently suffers some
change in direction as it passes into the water or
emerges therefrom. The passage of light from one
medium to another was studied by Snell in the
seventeenth century, and he enunciated the follow-
ing laws : —
1. The refracted ray lies in the plane containing
the incident ray and the normal to the plane surface
separating the two media.
It will be noticed that the reflected ray obeys
this law also.
2. The angle r, which the refracted ray makes
with the normal, is related to the angle z, which the
incident ray makes with the same direction, by the
equation
n sin i = n sin r, (a)
where n and n are constants for the two media
which are known as the indices of refraction, or the
refractive indices.
This simple trigonometrical relation may be ex-
pressed in geometrical language. Suppose we cut
a plane section through the two media at right
angles to the bounding plane, which then appears
as a straight line, SOS' (Fig. 14), and suppose that
IO represents the direction of the incident ray ; then
Snell's first law tells us that the refracted ray OR
will also lie in this plane. Draw the normal NON1,
and with centre O and any radius describe a circle
intersecting the incident and refracted rays in the
points a and b respectively ; let drop perpendiculars
ac and bd on to the normal NON'. Then we have
REFLECTION, REFRACTION, DISPERSION if
n .ac—ri . bd, whence we see that if n be greater than
«', ac is less than bd, and therefore when light passes
from one medium into another which is less optically
dense, in its passage across the boundary it is bent,
or refracted, away from the normal.
We see, then, that when light falls on the boundary
of two different media, some is reflected in the first
and some is refracted into the second medium.
N
FIG. 14.— Refraction across a Plane Surface.
The relative amounts of light reflected and refracted
depend on the angle of incidence and the refractive
indices of the media. We shall return to this point
when we come to consider the lustre of stones.
We will proceed to consider the course of rays at
different angles of incidence when light passes out
from a medium into one less dense — for instance,
from water into air. Corresponding to light with
a small angle of incidence such as I^O (Fig. 15),
some of it is reflected in the direction OI\ and the
2
i8
GEM-STONES
remainder is refracted out in the direction ORV
Similarly, for the ray 72<9 some is reflected along
0/2 and some refracted along ORZ. Since, in the
case we have taken, the angle of refraction is
greater than the angle of incidence, the refracted
ray corresponding to some incident, ray ICO will
graze the bounding surface, and corresponding to
Ic
FIG. 15. — Total -Reflection.
a ray beyond it, such as 73(9, which has a still greater
angle of incidence, there is no refracted ray, and
all the light is wholly or totally reflected within the
dense medium. The critical angle ICON, which is
called the angle of total-reflection, is very simply
related to the refractive indices of the two media ;
for, since r is now a right angle, sin r= i, and equa-
tion (a) becomes
n sin i — n' . (b\
REFLECTION, REFRACTION, DISPERSION 19
Hence, if the angle of total-reflection is measured
and one of the indices is known, the other can easily
be calculated.
The phenomenon of total-reflection may be ap-
preciated if we hold a glass of water above our head,
and view the light of a lamp on a table reflected
from the under surface of the water. This reflection
is incomparably more brilliant than that given by
the upper surface.
The refractive index of air is taken as unity ;
strictly, it is that of a vacuum, but the difference
is too small to be appreciated even in very delicate
work. Every substance has different indices for
light of different colour, and it is customary to take
as the standard the yellow light of a sodium flame.
This happens to be the colour to which our eyes are
most sensitive, and a flame of this kind is easily
prepared by volatilizing a little bicarbonate of soda
in the flame of a bunsen burner. A survey of
Table III at the end of the book shows clearly how
valuable a measurement of the refractive index is
for determining the species to which a cut stone
belongs. The values found for different specimens
of the species do in cases vary considerably owing
to the great latitude possible in the chemical con-
stitution due to the isomorphous replacement of one
element by another. Some variation in the index
may even occur in different directions within the
same stone ; it results from the remarkable
property of splitting up a beam of light into two
beams, which is possessed by many crystallized
substances. This forms the subject of a later
chapter.
Upon the fact that the refractive index of a
20 GEM-STONES
substance varies for light of different colours depends
such familiar phenomena as the splendour of the
rainbow and the ' fire ' of the diamond. When
white light is refracted into a stone it no longer
remains white, but is split up into a spectrum.
Except in certain anomalous substances the refractive
index increases progressively as the wave-length of
the light decreases, and consequently a normal
spectrum is violet at one end and passes through
green and yellow to red at the other end. The width
of the spectrum, which may be measured by the
difference between the refractive indices for the
extreme red and violet rays, also varies, though on
the whole it increases with the refractive index. It
is the dispersion, as this difference is termed, that
determines the ' fire ' — a character of the utmost
importance in colourless transparent stones, which,
but for it, would be lacking in interest. Diamond
excels all colourless stones in this respect, although
it is closely followed by zircon, the colour of which
has been driven off by heating ; it is, however, sur-
passed by two coloured species : sphene, which is
seldom seen in jewellery, and demantoid, the green
garnet from the Urals, which often passes under the
misnomer ' olivine.' The dispersion of the more
prominent species for the B and G lines of the solar
spectrum is given in Table IV at the end of the
book.
We will now proceed to discuss methods that
may be used for the measurement of the refractive
indices of cut stones.
CHAPTER IV
MEASUREMENT OF REFRACTIVE INDICES
THE methods available for the measurement
of refractive indices are of two kinds, and
make use of two different principles. The first,
which is based upon the very simple relation found
in the last chapter to subsist at total-reflection,
can be used with ease and celerity, and is best
suited for discriminative purposes ; but it is re-
stricted in its application. The second, which
depends on the measurement of the angle between
two facets and the minimum deviation experienced
by a ray of light when traversing a prism formed by
them, is more involved, entails the use of more
elaborate apparatus, and takes considerable time,
but it is less restricted in its application.
(i) THE METHOD OF TOTAL-REFLECTION
We see from equation b (p. 1 8), connecting the
angle of total-reflection with the refractive indices
of the adjacent media, that, if the denser medium
be constant, the indices of all less dense media
may be easily determined from a measurement of
the corresponding critical angle. In all refracto-
meters the constant medium is a glass with a high
refractive index. Some instruments have rotatory
22 GEM-STONES
parts, by means of which this angle is actually
measured. Such instruments give very good
results, but suffer from the disadvantages of being
neither portable nor convenient to handle, and of
not giving a result without some computation.
For use in the identification of cut stones, a
refractometer with a fixed scale, such as that (Fig.
1 6) devised by the author, is far more convenient.
In order to facilitate the observations, a totally
reflecting prism has been inserted between the two
FlG. 16. — Refractometer (actual size).
lenses of the eyepiece. The eyepiece may be
adjusted to suit the individual eyesight; but for
observers with exceptionally long sight an adapter
is provided, which permits the eyepiece being
drawn out to the requisite extent. The refracto-
meter must be held in the manner illustrated in
Fig. 17, so that the light from a window or other
source of illumination enters the instrument by the
lenticular opening underneath. Good, even illumina-
tion of the field may also very simply be secured
by reflecting light into the instrument from a sheet
REFRACTIVE INDICES 23
of white paper laid on a table. On looking down
the eyepiece we see a scale (Fig. 1 8), the eyepiece
being, if necessary, focused until the divisions of
the scale are clearly and distinctly seen. Suppose,
for experiment, we smear a little vaseline or similar
fatty substance on the plane surface of the dense
glass, which just projects beyond the level of the
FIG. 17. — Method of Using the Refractometer.
brass plate embracing it. The field of view is now
no longer uniformly illuminated, but is divided
into two parts (Fig. 19): a dark portion above,
which terminates in a curved edge, apparently
green in colour, and a bright portion underneath,
which is composed of totally reflected light. If
we tilt the instrument downwards so that light
enters the instrument from above through the
vaseline we find that the portions of the field are
GEM-STONES
reversed, the dark portion being underneath and
terminated by a red edge. It is possible so to
arrange the illumination that the two portions
are evenly lighted, and the common edge becomes
almost invisible. It is therefore essential for
obtaining satisfactory results that the plate and
the dense glass be shielded from the light by the
IEFFWCTIVE INDEX
-= 1-30
m 1-35
fH 1-40
fJH 1-45
= 1-50
HI 1-55
= 1-60
1-70
= 1-75
FIG. 1 8. —Scale
of the Refrac-
tometer.
FIG. 19.— Shadow,
edge given by a
singly refractive
Substance.
disengaged hand. The shadow-edge is curved,
and is, indeed, an arc of a circle, because spherical
surfaces are used in the optical arrangements of
the refractometer ; by the substitution of cylindrical
surfaces it becomes straight, but sufficient advantage
is not secured thereby to compensate for the greatly
increased complexity of the construction. The
shadow-edge is coloured, because the relative
dispersion, — (nv and nr being the refractive
REFRACTIVE INDICES 25
indices for the extreme violet and red rays
respectively), of the vaseline differs from that of
the dense glass. The dispersion of the glass is
very high, and exceeds that of any stone for
which it can be used. Certain oils have, however,
nearly the same relative dispersion, and the edges
corresponding to them are consequently almost
colourless. A careful eye will perceive that the
coloured shadow-edge is in reality a spectrum, of
which the violet end lies in the dark portion of
the field and the red edge merges into the bright
portion. The yellow colour of a sodium flame,
which, as has already been stated, is selected as
the standard for the measurement of refractive
indices, lies between the green and the red, and
the part of the spectrum to be noted is at the
bottom of the green, and practically, therefore, at
the bottom of the shadow, because the yellow and
red are almost lost in the brightness of the lower
portion of the field. If a sodium flame be used
as the source of illumination, the shadow-edge
becomes a sharply defined line. The scale is so
graduated and arranged that the reading where
this line crosses the scale gives the corresponding
refractive index, the reading, since the line is
curved, being taken in the middle of the field on
the right-hand side of the scale. The refracto-
meter therefore gives at once, without any inter-
mediate calculation, a value of the refractive index
to the second place of decimals, and a skilled
observer may, by estimating the tenths of the
intervals between successive divisions, arrive at
the third place ; to facilitate this estimation the
semi-divisions beyond 1-650 have been inserted.
26 GEM-STONES
The range extends nearly to r8oo; for any
substance with a higher refractive index the field
is dark as far as the limit at the bottom.
A fat, or a liquid, wets the glass, i.e, comes into
intimate contact with it, but if a solid substance
be tested in the same way, a film of air would
intervene and entirely prevent an observation. To
displace it, a drop of some liquid which is more
highly refractive than the substance under test
must first be applied to the plane surface of the
dense glass. The most convenient liquid for the
purpose is methylene iodide, CH2I2, which, when
pure, has at ordinary room temperatures a refrac-
tive index of 1*742. It is almost colourless when
fresh, but turns reddish brown on exposure to light.
If desired, it may be cleared in the manner described
below (p. 66), but the film of liquid actually used
is so thin that this precaution is scarcely necessary.
If we test a piece of ordinary glass — one of the slips
used by microscopists for covering thin sections is
very convenient for the purpose — first applying a
drop of methylene iodide to the plane surface of the
dense glass of the refractometer (Fig. 20), we notice
a coloured shadow-edge corresponding to the glass-
slip at about i'53O and another, almost colourless,
at 1742, which corresponds to the liquid. If the
solid substance which is tested is more highly
refractive than methylene iodide, only the latter
of the shadow-edges is visible, and we must utilize
some more refractive liquid. We can, however,
raise the refractive index of methylene iodide by
dissolving sulphur l in it ; the refractive index of
1 Methylene iodide must be heated almost to boiling-point to enable
it to absorb sufficient sulphur ; but caution must be exercised in the
REFRACTIVE INDICES 27
the saturated liquid lies well beyond i'8oo, and
the shadow-edge corresponding to it, therefore, does
not come within the range of the refractometer.
The pure and the saturated liquids can be procured
with the instrument, the bottles containing them
being japanned on the outside to exclude light and
fitted with dipping-stoppers, by means of which a
drop of the liquid required is easily transferred to
the surface of the glass of the instrument. So long
Stone
FIG. 20. — Faceted Stone in Position on the Refractometer.
as the liquid is more highly refractive than the stone,
or whatever may be the substance under examination,
its precise refractive index is of no consequence. The
facet used in the test must be flat, and must be
pressed firmly on the instrument, so that it is truly
parallel to the plane surface of the dense glass ; for
good results, moreover, it must be bright.
operation to prevent the liquid boiling over and catching fire, the
resulting fumes being far from pleasant. It is advisable to verify by
actual observation that the liquid is refractive enough not to show any
shadow-edge in the field of view of the refractometer.
28 GEM-STONES
We have so far assumed that the substance
which we are testing is simple and gives a
single shadow-edge; but, as may be seen from
Table V, many of the gem-stones are doubly
refractive, and such will, in general, show in the
field of the refractometer two distinct shadow-
edges more or less widely separated. Suppose,
for example, we study the effect produced by a
peridot, which displays the phenomenon to a
marked degree. If we revolve the stone so that
the facet under observation remains parallel to
the plane surface of the dense glass of the refracto-
meter and in contact with it, we notice that both
the shadow-edges in general move up or down
the scale. In particular cases, depending upon the
relation of the position of the facet selected to
the crystalline symmetry, one or both of them
may remain fixed, or one may even move across
the other. But whatever facet of the stone be
used for the test, and however variable be the
movements of the shadow -edges, the highest and
lowest readings obtainable remain the same; they
are the principal indices of refraction, such as are
stated in Table III at the end of the book, and
their difference measures the maximum amount of
double refraction possessed by the stone. The
procedure is therefore simplicity itself; we have
merely to revolve the stone on the instrument,
usually through not more than a right angle, and
note the greatest and least readings. It will be
noticed that the shadow-edges cross the scale
symmetrically in the critical and skewwise in
intermediate positions. Fig. 21 represents the
effect when the facet is such as to give simul-
REFRACTIVE INDICES
29
taneously the two readings required. The shadow-
edges a and b, which are coloured in white light,
correspond to the least and greatest respectively
of the principal refractive indices, while the third
shadow-edge, which is very faint, corresponds to
the liquid used — methylene iodide. It is possible,
as we shall see in a later chapter, to learn from
the motion, if any, of the shadow-
edges something as to the character
of the double refraction. Since,
however, each shadow-edge is spec-
tral in white light, they will not be
distinctly separate unless the double
refraction exceeds the relative dis-
persion. Topaz, for instance, ap-
pears in white light to yield only
a single shadow-edge, and may thus
easily be distinguished from tour-
maline, in which the double re-
fraction is large enough for the
separation of the two shadow-edges
to be clearly discerned. In sodium
light, however, no difficulty is ex-
perienced in distinguishing both the
shadow-edges given by substances with small amount
of double refraction, such as chrysoberyl, quartz, and
topaz, and a skilled observer may detect the separa-
tion in the extreme instances of apatite, idocrase,
and beryl. The shadow-edge corresponding to the
greater refractive index is always less distinct,
because it lies in the bright portion of the field.
If the stone or its facet be small, it must be moved
on the plane surface of the dense glass until the
greatest possible distinctness is imparted to the
FIG. 21.— Shadow-
edges given by a
doubly refractive
Substance.
30 GEM-STONES
edge or edges. If it be moved towards the
observer from the further end, a misty shadow
appears to move down the scale until the correct
position is reached, when the edges spring into
view.
Any facet of a stone may be utilized so long as
it is flat, but the table-facet is the most convenient,
because it is usually the largest, and it is available
even when the stone is mounted. That the stone
need not be removed from its setting is one of
the great advantages of this method. The smaller
the stone the more difficult it is to manipulate ;
caution especially must be exercised that it be
not tilted, not only because the shadow-edge would
be shifted from its true position and an erroneous
value of the refractive index obtained, but also
because a corner or edge of the stone would
inevitably scratch the glass of the instrument,
which is far softer than the hard gem-stones.
Methylene iodide will in time attack and stain the
glass, and must therefore be wiped off the instru-
ment immediately after use.
(2) THE METHOD OF MINIMUM DEVIATION
If the stone be too highly refractive for a
measurement of its refractive index to be possible
with the refractometer just described, and it is
desired to determine this constant, recourse must
be had to the prismatic method, for which purpose
an instrument known as a goniometer l is required.
1 yuvla, angle ; /j.4rpov, measure. For details of the construction,
adjustment, and use of this instrument the reader should refer to text-
books of mineralogy or crystallography.
REFRACTIVE INDICES 31
Two angles must be measured ; one the interior
angle included between a suitable pair of facets,
and the other the minimum amount of the deviation
produced by the pair upon a beam of light
traversing them.
Fig. 22 represents a section of a step-cut stone
perpendicular to a series of facets with parallel
edges ; / is the table, and a, b, c, are facets on
the culet side. The path of light traversing the
prism formed by the pair
of facets, / and b, is
indicated. Suppose that
A is the interior angle
of the prism, i the angle
of incidence of light at
the first facet and if the
angle of emergence at
the second facet, and r
and / the angles inside
the stone at the two facets
respectively. Then at the FIG. 22.— Path at Minimum De-
first facet light has been
bent through an angle a Cut Stone"."
i — r, and again at the
second facet through an angle i' - / ; the angle of
deviation, D, is therefore given by
We have further that
whence it follows that
A + D~i+?.
If the stone be mounted on the goniometer
32 GEM-STONES
and adjusted so that the edge of the prism is
parallel to the axis of rotation of the instrument
and if light from the collimator fall upon the
table-facet and the telescope be turned to the
proper position to receive the emergent beam, a
spectral image of the object-slit, or in the case of
a doubly refractive stone in general, two spectral
images, will be seen in white light ; in the light
of a sodium flame the images will be sharp and
distinct. Suppose that we rotate the stone in
the direction of diminishing deviation and simul-
taneously the telescope so as to retain an image in
the field of view, we find that the image moves
up to and then away from a certain position, at
which, therefore, the deviation is a minimum. The
image moves in the same direction from this
position whichever way the stone be rotated.
The question then arises what are the angles
of incidence and refraction under these special
conditions. It is clear that a path of light is
reversible ; that is to say, if a beam of light
traverses the prism from the facet t to the facet b,
it can take precisely the same path from the facet
b to the facet t. Hence we should be led to
expect that, since experiment teaches us that there
is only one position of minimum deviation corre-
sponding to the same pair of facets, the angles at the
two facets must be equal, i.e. i=if, and r—S. It
is, indeed, not difficult to prove by either geometrical
or analytical methods that such is the case.
^0
Therefore at minimum deviation r=— and
2
. A+D . .
t = , and, since sin t = n sm r, where « is
REFRACTIVE INDICES 33
the refractive index of the stone, we have the
simple relation —
This relation is strictly true only when the
direction of minimum deviation is one of crystal-
line symmetry in the stone, and holds therefore
in general for all singly refractive stones, and for
the ordinary ray of a uniaxial stone ; but the
values thus obtained even in the case of biaxial
stones are approximate enough for discriminative
purposes. If then the stone be singly refractive,
the result is the index required ; if it be uniaxial,
one value is the ordinary index and the other
image gives a value lying between the ordinary
and the extraordinary indices ; if it be biaxial, the
values given by the two images may lie anywhere
between the greatest and the least refractive indices.
The angle A must not be too large ; otherwise the
light will not emerge at the second facet, but will
be totally reflected inside the stone : on the other
hand, it must not be too small, because any error
in its determination would then seriously affect the
accuracy of the value derived for the refractive
index. Although the monochromatic light of a
sodium flame is essential for precise work, a
sufficiently approximate value for discriminative
purposes is obtained by noting the position of the
yellow portion of the spectral image given in white
light.
In the case of a stone such as that depicted in
Fig. 2 2 images are given by other pairs of facets, for
3
34
GEM-STONES
instance ta and tc, unless the angle included by
the former is too large. There might therefore be
some doubt, to which pair some particular image
corresponded; but no confusion can arise if the
following procedure be adopted.
The table, or some easily recognizable facet,
is selected as the facet at which light enters the
stone. The telescope is first placed in the position
in which it is directly opposite the collimator
(T0 in Fig. 23), and clamped. The scale is turned
until it reads ex-
actly zero, o° or
360°, and clamped.
The telescope is re-
leased and revolved
in the direction of
T* increasing readings
of the scale to the
position of minimum
deviation, T. The
reading of the scale
FIG. 23. — Course of Observations in the .
Method of Minimum Deviation. glves at once the
angle of minimum
deviation, D. The holder carrying the stone is
now clamped to the scale, and the telescope is
turned to the position, 7\, in which the image
given by reflection from the table facet is in the
centre of the field of view; the reading of the scale
is taken. The telescope is clamped, and the scale
is released and rotated until it reads the angle
already found for D. If no mistake has been made,
the reflected image from the second facet is now
in the field of view. It will probably not be quite
central, as theoretically it should be, because the
REFRACTIVE INDICES 35
stone may not have been originally quite in the
position of minimum deviation, a comparatively
large rotation of the stone producing no apparent
change in the position of the refracted image at
minimum deviation, and further, because, as has
already been stated, the method is not strictly true for
biaxial stones. The difference in readings, however,
should not exceed 2°. The reading, S, of the
scale is now taken, and it together with 180°
subtracted from the reading for the first facet, and
the value of A, the interior angle between the two
facets, obtained.
Let us take an example.
Reading T ( = /?) 40° 41' Reading 7\ 261° 35'
less 1 80° i So o
8i 35
Reading 5" 41 30
\D 20 2oJ A 40
\A 20 2\ \A 20
o 23
Log sin 40° 23' 9.81151
Log sin 20 2| 9.53492
Log n 0.27659
n= 1.8906.
The readings 5" and T are very nearly the same,
and therefore we may be sure that no mistake
has been made in the selection of the facets.
In place of logarithm-tables we may make use
of the diagram on Plate II. The radial lines
36 GEM-STONES
correspond to the angles of minimum deviation
and the skew lines to the prism angles, and the
distance along the radial lines gives the refractive
index. We run our eye along the line for the
observed angle of minimum deviation and note
where it meets the curve for the observed prism
angle ; the refractive index corresponding to the
point of intersection is at once read off.
This method has several obvious disadvantages :
it requires the use of an expensive and elaborate
instrument, an observation takes considerable time,
and the values of the principal refractive indices
cannot in general be immediately determined.
Table III at the end of the book gives the
refractive indices of the gem-stones.
Prism-angle
CHAPTER V
LUSTRE AND SHEEN
IT has been already stated that whenever light in
one medium falls upon the surface separating
it from another medium some of the light is
reflected within the first, while the remainder passes
out into the second medium, except when the first
is of lower refractivity than the second and light
falls at an angle greater than that of total-reflection.
Similarly, when light impinges upon a cut stone
some of it is reflected and the remainder passes into
the stone. What is the relative amount of reflected
light depends upon the nature of the stone — its
refractivity and hardness — and determines its
lustre ; the greater the amount the more lustrous
will the stone appear. There are different kinds of
lustre, and the intensity of each depends on the
polish of the surface. From a dull, i.e. an uneven,
surface the reflected light is scattered, and there are
no brilliant reflections. All gem-stones take a good
polish, and have therefore, so long as the surface
retains its polish, considerable brilliancy; turquoise, on
account of its softness, is always comparatively dull.
The different kinds of lustre are —
(1) Adamantine, characteristic of diamond.
(2) Vitreous, as seen on the surface of
fractured glass.
(3) Resinous, as shown by resins.
38 GEM-STONES
Zircon and demantoid, the green garnet called by
jewellers " olivine," alone among gem-stones have a
lustre approaching that of diamond. The remainder
all have a vitreous lustre, though varying in degree,
the harder and the more refractive species being on
the whole the more lustrous.
Some stones — for instance, a cinnamon garnet —
appear to have a certain greasiness in the lustre,
which is caused by stray reflections from inclusions
or other breaks in the homogeneity of the interior.
A pearly lustre, which arises from cleavage cracks
and is typically displayed by the cleavage face of
topaz, would be seen in a cut stone only when
flawed.
Certain corundums when viewed in the direction
of the crystallographical axis display six narrow
lines of light radiating at angles of 60° from a
centre in a manner suggestive of the conventional
representations of stars. Such stones are con-
sequently known as asterias, or more usually star-
stones — star-rubies or star-sapphires, as the case
may be, and the phenomenon is called asterism.
These stones have not a homogeneous structure,
but contain tube-like cavities regularly arranged
at angles of 60* in planes at right angles to the
crystallographical axis. The effect is best produced
when the stones are cut en cabochon perpendicular
to that axis.
Chatoyancy is a somewhat similar phenomenon,
but in this case the fibres or cavities are parallel
to a single direction, and a single broadish band
is displayed at right angles to it. Cat's-eyes, as
these stones are termed, are cut en cabochon parallel to
the fibres. The true cat's-eye (Plate XXIX, Fig. i)
LUSTRE AND SHEEN 39
is a variety of chrysoberyl, but the term is also
often applied to quartz showing a similar appearance.
The latter is really a fibrous mineral, such as
asbestos, which has become converted into silica.
The beautiiul tiger's-eye from South Africa is a
silicified crocidolite, the original blue colour of which
has been altered by oxidation to golden brown.
Recently tourmalines have been discovered which
are sufficiently fibrous in structure to display an
effective chatoyancy.
The milky sheen of moonstone (Plate XXIX,
Fig. 4) owes its effect to reflections from twin
lamellae. The wonderful iridescence which is the
glory of opal, and is therefore termed opalescence,
arises from a structure which is peculiar to that
species. Opal is a solidified jelly ; on cooling it
has become riddled with extremely thin cracks,
which were subsequently filled with similar material
of slightly different refractivity, and thus it consists
of a series of films. At the surface of each film
interference of light takes place just as at the surface
of a soap-bubble, and the more evenly the films are
spaced apart the more uniform is the colour displayed,
the actual tint depending upon the thickness of the
films traversed by the light giving rise to the
phenomenon.
CHAPTER VI
DOUBLE REFRACTION
r I ^HE optical phenomenon presented by many
J. gem-stones is complicated by their property
of splitting up a beam of light into two with, in
general, differing characters. In this chapter we
shall discuss the nature of double refraction, as it is
termed, and methods for its detection. The pheno-
menon is not one that comes within the purview of
everyday experience.
So long ago as 1669 a Danish physician, by
name Bartholinus, noticed that a plate of the trans-
parent mineral which at that time had recently been
brought over from Iceland, and was therefore called
" Iceland-spar," possessed the remarkable property
of giving a double image of objects close to it when
viewed through it. Subsequent investigation has
shown that much crystallized matter is doubly
refractive, but in calcite — to use the scientific name
for the species which includes Iceland-spar — alone
among common minerals is the phenomenon so
conspicuous as to be obvious to the unaided eye.
The apparent separation of the pair of images given
by a plate cut or cleaved in any direction depends
upon its thickness. The large mass, upwards of
two feet (60 cm.) in thickness, which is exhibited
at the far end of the Mineral Gallery of the British
DOUBLE REFRACTION 41
Museum (Natural History), displays the separation
to a degree that is probably unique.
Although none of the gem-stones can emulate
calcite in this character, yet the double refraction
of certain of them is large enough to be detected
without much difficulty. In the case of faceted
stones the opposite edges should be viewed through
the table-facet, and any signs of doubling noted.
FIG. 24. — Apparent doubling of the Edges of a Peridot when
viewed through the Table-Facet.
The double refraction of sphene is so large, viz.
O'O8, that the doubling of the edges is evident to
the unaided eye. In peridot (Fig. 24), zircon (b),
and epidote the apparent separation of the edges is
easily discerned with the assistance of an ordinary
lens. A keen eye can detect the phenomenon even
in the case of such substances as quartz with small
double refraction. It must, however, be remembered
that in all such stones the refraction is single in
certain directions, and the amount of double refraction
42 GEM-STONES
varies therefore with the direction from nil to the
maximum possessed by the stone. Experiment
with a plate of Iceland-spar shows that the rays
transmitted by it have properties differing from
those of ordinary light On superposing a second
plate we notice that there are now two pairs of
images, which are in general no longer of equal
brightness, as was the case before. If the second
plate be rotated with respect to the first, two images,
one of each pair, disappear, and then the other two,
the plate having turned through a right angle
between the two positions of extinction ; midway
between these positions the images are all equally
a
FIG. 25. — Wave-Motion.
bright This variation of intensity implies that
each of the rays emerging from the first plate has
acquired a one-sided character, or, as it is usually
expressed, has become plane-polarized, or, shortly,
polarized.
Before the discovery of the phenomenon of double
refraction the foundation of the modern theory of
light had been laid by the genius of Huygens.
According to this theory light is the result of a
wave-motion (Fig. 25) in the ether, a medium that
pervades the whole of space whether occupied by
matter or not, and transmits the wave-motion at a
rate varying with the matter with which it happens
to coincide. Such a medium has been assumed
DOUBLE REFRACTION 43
because it explains satisfactorily all the phenomena
of light, but it by no means follows that it has a
concrete existence. Indeed, if it has, it is so
tenuous as to be imperceptible to the most delicate
experiments. The wave-motion is similar to that
observed on the surface of still water when disturbed
by a stone flung into it. The waves spread out
from the source of disturbance; but, although the
waves seem to advance, the actual particles of water
merely move up and down, and have no motion at
all in the direction in which the waves are moving.
If we imagine similar motion to take place in any
plane and not only the horizontal, we form some idea
of the nature of ordinary light. But after passing
through a plate of Iceland-spar, light no longer
vibrates in all directions, but in each beam the
vibrations are parallel to a particular plane, the two
planes being at right angles. The exact relation of
the direction of the vibrations to the plane of polariz-
ation is uncertain, although it undoubtedly lies in the
plane containing the direction of the ray of light and
the perpendicular to the plane of polarization. The
waves for different colours differ in their length, i.e.
in the distance, 2 bb (Fig. 25), from crest to crest,
while the velocity, which remains the same for the
same medium, is proportional to the wave-length.
The intensity of the light varies as the square of the
amplitude of the wave, i.e. the height, ab, of the
crest from the mean level.
Various methods have been proposed for obtain-
ing polarized light. Thus Seebeck found in 1813
that a plate of brown tourmaline cut parallel to the
crystallographic axis and of sufficient thickness
(cf. p. n) transmits only one ray, the other being
44 GEM-STONES
entirely absorbed within the plate. Another method
was to employ a glass plate to reflect light at a
certain critical angle. The most efficient method,
and that in general use at the present day, is due
to the invention of Nicol. A rhomb of Iceland-
spar (Fig. 26), of suitable length, is sliced along the
longer diagonal, dd, and the halves are cemented
together by means of Canada balsam. One ray,
ioo, is totally reflected at the surface separating the
mineral and the cement, and does not penetrate
into the other half; while the other ray, iee, is trans-
mitted with almost undiminished intensity. Such
FIG. 26.— Nicol's Prism.
a rhomb is called a Nicol's prism after its inventor,
or briefly, a nicol.
If one nicol be placed above another and their
corresponding principal planes be at right angles
no light is transmitted through the pair. In the
polarizing microscope one such nicol, called the
polarizer, is placed below the stage, and the other,
called the analyser, is either inserted in the body
of the microscope or placed above the eyepiece, and
the pair are usually set in the crossed position so
that the field of the microscope is dark. If a piece
of glass or a fragment of some singly refractive sub-
stance be placed on the stage the field still remains
DOUBLE REFRACTION 45
dark ; but in case of a doubly refractive stone the
field is no longer dark except in certain positions
of the stone. On rotation of the plate, or, if
possible, of the nicols together, the field passes from
darkness to maximum brightness four times in a
complete revolution, the relative angular intervals
between these positions being right angles. These
positions of darkness are known as the positions of
extinction, and the plate is said to extinguish in
them. This test is exceedingly delicate and reveals
the double refraction even when the greatest
difference in the refractive indices is too small to
be measured directly.
Doubly refractive substances are of two kinds:
uniaxial, in which there is one direction of single
refraction, and biaxial, in which there are two such
directions. In the case of the former the direction
of one, the ordinary ray, is precisely the same as if
the refraction were single, but the refractive index
of the other ray varies from that of the ordinary
ray to a second limiting value, the extraordinary
refractive index, which may be either greater or less.
If the extraordinary is greater than the ordinary
refractive index the double refraction is said to be
positive ; if less, to be negative. A biaxial substance
is more complex. It possesses three principal
directions, viz., the bisectrices of the directions of
single refraction and the perpendicular to the plane
containing them. The first two correspond to the
greatest and least, and the last to the mean of the
principal indices of refraction. If the acute
bisectrix corresponds to the least refractive index,
the double refraction is said to be positive, and if to
the greatest, negative. The relation of the .direc-
GEM-STONES
tions of single refraction, s, to the three principal
directions, a, b, c, is illustrated in Fig. 27 for the
case of topaz, a positive mineral. The refractive
indices of the rays traversing one -of the principal
directions have the values corresponding to the
other two. In the direction a we should measure
the greatest and the mean of the principal refractive
indices, in the direction b the greatest and the least,
and in the direction c the mean and the least. The
maximum amount of double refraction is there-
fore in the direction b.
In the examination of a
faceted stone, of the most
usual shape, the simplest
method is to lay the large
facet, called the table, on a
-b glass slip and view the stone
through the small parallel
facet, the culet. Should the
FIG. 27,-Relation of the latter not exist> * mav fre-
two Directions of single quently happen that owing
Refraction to the prin- to internal reflection no light
emerges through the steeply
inclined facets. This difficulty
is easily overcome by immersing the stone in some
highly refracting oil. A glass plate held by hand
over the stone with a drop of the oil between it
and the plate serves the purpose, and is perhaps a
more convenient method. A stone which does not
possess a pair of parallel facets should be viewed
through any pair which are nearly parallel.
We have stated that a plate of glass has no effect
on the field. Suppose, however, it were viewed
when placed between the jaws of a tightened vice
DOUBLE REFRACTION 47
and thus thrown into a state of strain, it would then
show double refraction, the amount of which would
depend on the strain. Natural singly refractive
substances frequently show phenomena of a similar
kind. Thus diamond sometimes contains a drop
of liquid carbonic acid, and the strain is revealed
by the coloured rings surrounding the cavity which
are seen when the stone is viewed between crossed
nicols. Double refraction is also common in
diamond even when there is no included matter to
explain it, and is caused by the state of strain into
which the mineral is thrown on release from the
enormous pressure under which it was formed.
Other minerals which display these so-called optical
anomalies, such as fluor and garnet, are not really
quite singly refractive at ordinary temperatures ;
each crystal is composed of several double refractive
individuals. But all such phenomena cannot be
confused with the characters of minerals which ex-
tinguish in the ordinary way, since the stone will
extinguish in small patches and these will not be
dark all at the same time ; further, the double re-
fraction is small, and on revolving the stone between
crossed nicols the extinction is not sharp. Paste
stones are sometimes in a state of strain, and
display slight, but general, double refraction.
Hence the existence of double refraction does not
necessarily prove that the stone is real and not an
imitation. Stones may be composed of two or
more individuals which are related to each other
by twinning, in which case each individual would
in general extinguish separately. Such individuals
would be larger and would extinguish more sharply
than the patches of an anomalous stone.
48
GEM-STONES
An examination in convergent light is sometimes
of service. An auxiliary lens is placed over the
condenser so as to converge the light on to the stone.
Light now traverses the stone in different directions ;
the more oblique the direction the greater the
distance traversed in the stone. If it be doubly
refractive, in any given direction there will be in
general two rays with differing refractive indices and
the resulting effect is akin to the well-known
phenomenon of New-
ton's rings, and is an
instance of what is
termed interference.
It may be mentioned
that the interference
of light (Fig. 28)
explains such com-
mon phenomena as
the colours of a
soap-bubble, the hues
of tarnished steel, the tints of a layer of oil floating
on water, and so on. Light, after diverging from
the stone, comes to focus a little beneath the plane
in which the image of the stone is formed. An
auxiliary lens must, therefore, be inserted to bring
the focal planes together, so that the interference
picture may be viewed by means of the same eye-
piece.
If a uniaxial crystal be examined along the
crystallographic axis in convergent light an inter-
ference picture will be seen of the kind illustrated on
Plate III. The arms of a black cross meet in the
centre of the field, which is surrounded by a series
of circular rings, coloured in white light. Rotation
FIG. 28.— Interference of Light.
I. UNIAXIAL
INTERFERENCE FIGURES
DOUBLE REFRACTION 49
of the stone about the axis produces no change in
the picture.
A biaxial substance possesses two directions (the
optic axes] along which a single beam is transmitted.
If such a stone be examined along the line bisecting
the acute angle between the optic axes (the acute
bisectrix] an interference picture l will be seen which
in particular positions of the stone with respect to
the crossed nicols takes the forms illustrated on
Plate III. As before, there is a series of rings
which are coloured in white light ; they, however,
are no longer circles but consist of curves known
as lemniscates, of which the figure of 8 is a special
form. Instead of an unchangeable cross there are
a pair of black " brushes " which in one position of
the stone are hyperbolae, and in that at right angles
become a cross. On rotating the stone we find
that the rings move with it and are unaltered in
form, whereas the brushes revolve about two points,
called the " eyes," where the optic axes emerge. If
the observation were made along the obtuse bisectrix
the angle between the optic axes would probably
be too large for the brushes to come into the field,
and the rings might not be visible in white light,
though they would appear in monochromatic light.
In the case of a substance like sphene the figure is
not so simple, because the positions of the optic
axes vary greatly for the different colours and the
result is exceedingly complex ; in monochromatic
light, however, the usual figure is visible.
It would probably not be possible in the case of
1 A cleavage flake of topaz may conveniently be used to show the
phenomenon, but owing to the great width of the angle the "eyes"
are invisible.
50 GEM-STONES
a faceted stone to find a pair of faces perpendicular
to the required direction. Nevertheless, so long as
a portion of the figures described is in the field of
view, the character of the double refraction, whether
uniaxial or biaxial, may readily be determined.
There is yet another remarkable phenomenon
which must not be passed over. Certain substances,
of which quartz is a conspicuous example and in
this respect unique among the gem-stones, possess
the remarkable property of rotating the plane of
polarization of a ray of light which is transmitted
parallel to the optic axis. If a plate of quartz be
cut at right angles to the axis and placed between
crossed nicols in white light, the field will be
coloured, the hue changing on rotation of one nicol
with respect to the other. Examination in
monochromatic light shows that the field will
become dark after a certain rotation of the one
nicol with respect to the other, the amount of which
depends on the thickness of the plate. If the plate
be viewed in convergent light, an interference picture
is seen as illustrated on Plate III, which is similar to,
and yet differs in some important particulars from
the ordinary interference picture of a uniaxial stone.
The cross does not penetrate beyond the innermost
ring and the centre of the field is coloured in white
light. If a stone shows such a picture, it may be
safely assumed to be quartz. It is interesting to
note that minerals which possess this property have
a spiral arrangement of the constituent atoms.
It has already been remarked (p. 28) that if a
faceted doubly refractive stone be rotated with one
facet always in contact with the dense glass of the
refractometer the pair of shadow-edges that are
DOUBLE REFRACTION 51
visible in the field move up or down the scale in
general from or to maximum and minimum
positions. The manner in which this movement
takes place depends upon the character of the
double refraction and the position of the facet under
observation with regard to the optical symmetry of
the stone. In the case of a uniaxial stone, if the
facet be perpendicular to the crystallographic axis,
i.e. the direction of single refraction, neither of the
shadow-edges will move. If the facet be parallel
to that direction, one shadow-edge will move up and
coincide with the other, which remains invariable
in position, and away from it to a second critical
position ; the latter gives the value of the extra-
ordinary refractive index, and the invariable shadow-
edge corresponds to the ordinary refractive index.
This phenomenon is displayed by the table-facet 01
most tourmalines, because for reasons given above
(p. 11) they are as a rule cut parallel to the
crystallographic axis. In the case of facets in
intermediate positions, the shadow-edge correspond-
ing to the extraordinary refractive index moves, but
not to coincidence with the invariable shadow-edge.
The case of a biaxial stone is more complex. If
the facet be perpendicular to one of the principal
directions one shadow-edge remains invariable in
position, corresponding to one of the principal
refractive indices, whilst the other moves between
the critical values corresponding to the remaining
two of the principal refractive indices. In the
interesting case in which the facet is parallel to the
two directions of single refraction, the second shadow-
edge moves across the one which is invariable in
position. In intermediate positions of the facet both
52 GEM-STONES
shadow-edges move, and give therefore critical values.
Of the intermediate pair, i.e. the lower maximum and
the higher minimum, one corresponds to the mean
principal refractive index, and the other depends
upon the relation of the facet to the optical
symmetry. If it is desired to distinguish between
them, observations must be made on a second facet ;
but for discriminative purposes such exactitude is
unnecessary, since the least and the greatest refractive
indices are all that are required.
The character of the refraction of gem-stones is
given in Table V at the end of the book.
CHAPTER VII
ABSORPTION EFFECTS: COLOUR, DICHROISM,
ETC.
WHEN white light passes through a cut stone,
colour effects result which arise from a
variety of causes. The most obvious is the funda-
mental colour of the stone, which is due to its
selective absorption of the light passing through it,
and would characterize it before it was cut. Inter-
mingled with the colour in a transparent stone is
the dispersive effect known as 'fire,' which has
already been discussed (p. 20). In many instances
the want of homogeneity is responsible for some
peculiar effects such as opalescence, chatoyancy, and
asterism. These phenomena will now be considered
in fuller detail.
COLOUR
All substances absorb light to some extent. If
the action is slight and affects equally the whole of
the visible spectrum, the stone appears white or
colourless. Usually some portion is more strongly
absorbed than the rest, and the stone seems to be
coloured. What is the precise tint depends not
only upon the portions transmitted through the
stone, but also upon their relative intensities. The
eye, unlike the ear, has not the power of analysis
54 GEM-STONES
and it cannot of itself determine how a composite
colour has been made up. Indeed, so far as it is
concerned, any colour may be exactly matched by
compounding in certain proportions three simple
primary colours — red, yellow, and violet. Alex-
andrite, a variety of chrysoberyl, is a curious and
instructive case. The balance in the spectrum of
light transmitted through it is such that, whereas in
daylight such stones appear green, in artificial light,
especially in gas-light, they are a pronounced
raspberry-red (Plate XXVII, Figs, n, 13). The
phenomenon is intensified by the strong dichroism
characteristic of this species.
The colour is the least reliable character that may
be employed for the identification of a stone, since it
varies considerably in the same species, and often
results from the admixture of some metallic oxide,
which has no essential part in the chemical com-
position and is present in such minute quantities
as to be almost imperceptible by analysis. Who
would, for instance, imagine from their appearance
that stones so markedly diverse in hue as ruby
and sapphire were really varieties of the same species,
corundum ? Again, quartz, in spite of the simplicity
of its composition, displays extreme differences of
tint. Nevertheless, certain varieties do possess a
distinctive colour, emerald being the most striking
example, and in other cases the trained eye can
appreciate certain characteristic subtleties of shade.
At any rate, the colour is the most obvious of the
physical characters, and serves to provide a rough
division of the species, and accordingly in Table II
at the end of the book the gem-stones are arranged
by their usual tints.
ABSORPTION EFFECTS
55
DlCHROISM
The two rays into which a doubly refractive stone
splits up a ray of light are often differently absorbed
by it, and in consequence appear on emergence
differently coloured ; such stones are said to be
dichroic. The most striking instance is a deep-
brown tourmaline, which, except in very thin
sections, is quite opaque to the ordinary ray. The
light transmitted by a plate cut parallel to the
FIG. 29. — Dichroscope (actual size).
crystallographic axis is therefore plane-polarized ;
before the invention by Nicol of the prism of Iceland-
spar known by his name this was the ordinary
method of obtaining light of this character (cf. p. 43).
Again, in the case of kunzite and cordierite the
difference in colour is so marked as to be obvious to
the unaided eye ; but where the contrast is less
pronounced we require the use of an instrument
called a dichroscope, which enables the twin colours
to be seen side by side.
Fig. 29 illustrates in section the construction of a
dichroscope. The instrument consists essentially of
5 6 GEM-STONES
a rhomb of Iceland-spar, S, of such a length as to
give two contiguous images (Fig. 30) of a square
hole, //, in one end of the tube containing it. In
some instruments the terminal faces of the rhomb
are ground at right angles to its
length, but usually, as in that
depicted, prisms of glass, G, are
cemented on to the two ends. A
CEP C> with a sli£htly lar£er h°le>
FIG. 30. -Field of ...... , ^
the Dichroscope. which is circular in shape, fits on
the end of the tube, and can be
moved up and down it and revolved round it, as
desired. The stone, R, to be tested may be
directly attached to it by means of some kind of
wax or cement in such a way that light which has
traversed it passes into the window, H, of the in-
strument ; the cap at the same time permits of
the rotation of the stone about the axis of the main
tube of the instrument. The dichroscope shown in
the figure has a still more convenient arrangement :
it is provided with an additional attachment, A, by
means of which the stone can be turned about an
axis at right angles to the length of the tube, and
thus examined in different directions. At the other
end of the main tube is placed a lens, L, of low
power for viewing the twin images : the short tube
containing it can be pushed in and out for focusing
purposes. Many makers now place the rhomb close
to the lens, L, and thereby require a much smaller
piece of spar ; material suitable for optical purposes
is fast growing scarce.
Suppose that a plate of tourmaline cut parallel to
its crystallographic axis is fastened to the cap and
the latter rotated. We should notice, on looking
ABSORPTION EFFECTS 57
through the instrument, that in the course of a
complete revolution there are two positions, ori-
entated at right angles to one another, in which
the tints of the two images are identical, the
positions of greatest contrast of tint being midway
between. If we examine a uniaxial stone in a
direction at right angles to its optic axis we obtain
the colours corresponding to the ordinary and the
extraordinary rays. In any direction less inclined
to the axis we still have the colour for the ordinary
ray, but the other colour is intermediate in tint
between it and that for the extraordinary ray. The
phenomenon presented by a biaxial stone is more
complex. There are three principal colours which
are visible in differing pairs in the three principal
optical directions ; in other directions the tints seen
are intermediate between the principal colours.
Since biaxial stones have three principal colours,
they are sometimes said to be trichroic or pleochroic,
but in any single direction they have two twin
colours and show dichroism. No difference at all
will be shown in directions in which a stone is
singly refractive, and it is therefore always advisable
to examine a stone in more than one direction lest
the first happens to be one of single refraction. For
determinative purposes it is __not ijiecessary to note
the exact shades of tint of the twin colours, because
they vary with the inherent colour of the stone, and
are therefore not constant for the same species ; we
need only observe, when the stone is tested with the
dichroscope, whether there is any variation of colour,
and, if so, its strength. Dichroism is a result of
double refraction, and cannot exist in a singly
refractive stone. The converse, however, is not true
58 GEM-STONES
and it by no means follows that, because no dichroism
can be detected in a stone, it is singly refractive. A
colourless stone, for instance, cannot possibly be
dichroic, and many coloured, doubly refractive
stones — for example, zircon — exhibit no dichroism, or
so little that it is imperceptible. The character is
always the better displayed, the deeper the inherent
colour of the stone. The deep-green alexandrite,
for instance, is far more dichroic than the lighter
coloured varieties of chrysoberyl.
If the stone is attached to the cap of the
instrument, the table should be turned towards it so
as to assure that the light passing into the instru-
ment has actually traversed the stone. If little
light enters through the opposite coign, a drop of oil
placed thereon will overcome the difficulty (cf. p. 46).
It is also necessary, for reasons mentioned above, to
examine the stone in directions as far as possible
across the girdle also. A convenient, though not
strictly accurate, method is to lay the stone with the
table facet on a table and examine the light which
has entered the stone and been reflected at that
facet. The stone may easily be rotated on the
table, and observations thus made in different
directions in the stone. Care must be exercised
in the case of a faceted stone not to mistake the
alteration in colour due to dispersion for a dichroic
effect, and the stone must be placed close to the instru-
ment during an observation, because otherwise the
twin rays traversing the instrument may have taken
sensibly different directions in the stone.
Dichroism is an effective test in the case of ruby;
its twin colours — purplish and yellowish red — are
in marked contrast, and readily distinguish it from
ABSORPTION EFFECTS 59
other red stones. Again, one of the twin colours
of sapphire is distinctly more yellowish than the
other ; the blue spinel, of which a good many have
been manufactured during recent years, is singly
refractive, and, of course, shows no difference of tint
in the dichroscope.
Table VI at the end of the book gives the
strength of the dichroism of the gem-stones.
ABSORPTION SPECTRA
A study of the chromatic character of the light
transmitted by a coloured stone is of no little
interest. As was stated above, the eye has not the
power of analysing light, and to resolve the trans-
mitted rays into their component parts an instru-
ment known as a spectroscope is needed. The
small ' direct-vision ' type has ample dispersion for
this purpose. It is advantageous to employ by
preference the diffraction rather than the prism
form, because in the former the intervals in the
resulting spectrum corresponding to equal differences
of wave-length are the same, whereas in the latter
they diminish as the wave-length increases and
accordingly the red end of the spectrum is relatively
cramped.
The absorptive properties of all doubly refractive
coloured substances vary more or less with the
direction in which light traverses them according to
the amount of dichroism that they possess, but the
variation is not very noticeable unless the stone is
highly dichroic. If the light transmitted by a deep-
coloured ruby be examined with a spectroscope it
will be found that the whole of the green portion
6o
GEM-STONES
of the spectrum is obliterated (Fig. 31), while in the
case of a sapphire only a small portion of the red
end of the spectrum is absorbed. Alexandrite
affords especial interest. In the spectrum of the
ALMANDINE
ALEXANDRITE
RUBY
THE SOLAR SPECTRUM
FIG. 31.— Absorption Spectra.
light transmitted by it, the violet and the yellow are
more or less strongly absorbed, depending upon the
direction in which the rays have passed through the
stone (Fig. 31), and the transmitted light is mainly
composed of two portions — red and green. The
apparent colour of the stone depends, therefore, upon
ABSORPTION EFFECTS 61
which of the two predominates. In daylight the
resultant colour is green flecked with red and orange,
the three principal absorptive tints (cf. p. 235), but in
artificial light, which is relatively stronger in the red
portion of the spectrum, the resultant colour is a
raspberry-red, and there is less apparent difference in
the absorptive tints (cf. Plate XXVII, Figs. 1 1, 13).
In all the spectra just considered, and in all like
them, the portions that are absorbed are wide, the
passage from blackness to colour is gradual, and
the edges deliminating them are blurred. In the
spectra of certain zircons and in almandine garnet
the absorbed portions, or bands as they are called,
are narrow, and, moreover, the transition from black-
ness to colour is sharp and abrupt ; such stones are
therefore said to display absorption-bands. Church
in 1866 was the first to notice the bands shown by
zircon (Fig. 31). Sorby thought they portended the
existence of a new element, to which he gave the
name jargonium, but subsequently discovered that
they were caused by the presence of a minute trace
of uranium. A yellowish-green zircon shows the
phenomenon best, and it has all the bands shown
in the figure. The spectrum varies slightly but
almost imperceptibly with the direction in the stone.
Others show the bands in the yellow and green,
while others show only those in the red, and some
only one of them. The bands are not confined to
stones of any particular colour, or amount of double
refraction. Again, many zircons show no bands at
all, so that their absence by no means precludes the
stone from being a zircon.
Almandine is characterized by a different spectrum
(Fig. 31). The band in the yellow is the most con-
62 GEM-STONES
spicuous, and is no doubt responsible for the purple
hue of a typical almandine. The spectrum varies
in strength in different stones. Rhodolite (p. 2 14), a
garnet lying between almandine and pyrope, displays
the same bands, and indications of them may be
detected in the spectra of pyropes of high refraction.
JEWELLERY DESIGNS
64 GEM-STONES
species, and is therefore very useful for discrimina-
tive purposes. It can be determined whatever be
the shape of the stone, and it is immaterial whether
it be transparent or not ; but, on the other hand, the
stone must be unmounted and free from the setting.
The methods for the determination of the specific
gravity are of two kinds : in the first a liquid is
found of the same, or nearly the same, density as
the stone, and in the second weighings are made
and the use of an accurate balance is required.
(i) HEAVY LIQUIDS
Experiment tells us that a solid substance floats in
a liquid denser than itself, sinks in one less dense,
and remains suspended at any level in one of pre-
cisely the same density. If the stone be only
slightly less dense than the liquid, it will rise to the
surface ; if it be just as slightly denser, it will as
surely sink to the bottom, a physical fact which has
added so much to the difficulty and danger of sub-
marine manoeuvring. If then we can find a liquid
denser than the stone to be tested, and place the
latter in it, the stone will float on the surface. If we
take a liquid which is less dense than the stone and
capable of mixing with the heavier liquid, and add
it to the latter, drop by drop, gently stirring so as
to assure that the density of the combination is
uniformly the same throughout, a stage is finally
reached when the stone begins to move downwards.
It has now very nearly the density of the liquid,
and, if we find by some means this density, we
know simultaneously the specific gravity of the
stone.
SPECIFIC GRAVITY 65
Various devices and methods are available for
ascertaining the density of liquids — for instance,
Westphal's balance ; but, apart from the incon-
venience attending such a determination, the density
of all liquids is somewhat seriously affected by
changes in the temperature, and it is therefore better
to make direct comparison with fragments of sub-
stances of known specific gravity, which are termed
indicators. If of two fragments differing slightly in
specific gravity one floats on the surface of a uniform
column of liquid and the other lies at the bottom of
the tube containing the liquid, we may be certain
that the density of the liquid is intermediate between
the two specific gravities. Such a precaution is
necessary because, if the liquid be a mixture of two
distinct liquids, the density would tend to increase
owing to the greater volatility of the lighter of them,
and in any case the density is affected by change of
temperature. The specific gravity of stones is not
much altered by variation in the temperature.
A more convenient variation of this method is to
form a diffusion column, so that the density increases
progressively with the depth. If the stone under
test floats at a certain level in such a column inter-
mediate between two fragments of known specific
gravity, its specific gravity may be found by
elementary interpolation. To form a column of
this kind the lighter liquid should be poured on to
the top of the heavier. Natural diffusion gives
the most perfect column, but, being a lengthy
process, it may conveniently be quickened by gently
shaking the tube, and the column thus formed
gives results sufficiently accurate for discriminative
purposes.
5
66 GEM-STONES
By far the most convenient liquid for ordinary
use is methylene iodide, which has already been
recommended for its high refraction. It has, when
pure, a density at ordinary room-temperatures of
3'324, and it is miscible in all proportions with
benzol, whose density is cr88, or toluol, another
hydrocarbon which is somewhat less volatile than
benzol, and whose density is about the same, namely,
O'86. When fresh, methylene iodide has only a
slight tinge of yellow, but it rapidly darkens on
exposure to light owing to the liberation of iodine
which is in a colloidal form and cannot be removed
by filtration, The liquid may, however, be easily
cleared by shaking it up with any substance with
which the iodine combines to form an iodide remov-
able by filtration. Copper filings answer the purpose
well, though rather slow in action ; mercury may
also be used, but is not very satisfactory, because
a small amount may be dissolved and afterwards be
precipitated on to the stone under test, carrying it
down to the bottom of the tube. Caustic potash
(potassium hydroxide) is also recommended ; in this
case the operation should preferably be carried out
in a special apparatus which permits the clear liquid
to be drawn off underneath, because water separates
out and floats on the surface. In Fig. 32 three cut
stones, a quartz (ft), a beryl (£), and a tourmaline (c)
are shown floating in a diffusion column of methy-
lene iodide and benzol. Although the beryl is only
slightly denser than the quartz, it floats at a
perceptibly lower level. These three species are
occasionally found as yellow stones of very similar
tint.
Various other liquids have been used or proposed
SPECIFIC GRAVITY
for the same purpose, of which two may be
mentioned. The first of them is a saturated solu-
tion of potassium iodide and mercuric iodide in
water, which is known after the
discoverer as Sonstadt's solution.
It is a clear mobile liquid with an
amber colour, having at 12° C. a
density of 3*085 ; it may be mixed
with water to any extent, and is
easily concentrated by heating;
moreover, it is durable and not sub-
ject to alteration of any kind ; on
the other hand, it is highly poisonous
and cauterizes the skin, not being
checked by albumen ; it also de-
stroys brass-ware by amalgamating FlG> 32._Stones
the metal. The second is Klein's of different Spe-
solution, a clear yellow liquid which cific Gravities
has at 1 5' C. a density of 3-28. It S?£2
Consists of the boro - tungState Of of heavy Liquid.
cadmium, of which the formula is
9WO3.B2O3.2CdO.2H2O+i6Aq, dissolved in water,
with which it may be diluted. If the salt be heated,
it fuses at 75° C. in its own water of crystallization
to a yellow liquid, very mobile, with a density of
3-55. Klein's solution is harmless, but it cannot
compare for convenience of manipulation with
methylene iodide.
The most convenient procedure is to have at
hand three glass tubes, fitted with stoppers or corks,
to contain liquids of different densities —
(a) Methylene iodide reduced to 27 ; using as
indicators orthoclase 2*55, quartz 2*66, and beryl
274.
68 GEM-STONES
(£) Methylene iodide reduced to 3-1 ; indicators,
beryl 2*74 and tourmaline 3*10.
(c) Methylene iodide, undiluted, 3'32.
The pure liquid in the last tube should on no
account be diluted ; but the density of the other
two liquids may be varied slightly, either by adding
benzol in order to lower it, or by allowing benzol,
which has far greater volatility than methylene
iodide, to evaporate, or by adding methylene iodide,
in order to increase it. The density of the liquids
may be ascertained approximately from the in-
dicators.
A glance at the table of specific gravities shows
that as regards the gem-stones methylene iodide is
restricted in its application, since it can be used to
test only moonstone, quartz, beryl, tourmaline, and
spodumene; opal and turquoise, being amorphous
and more or less porous, should not be immersed
in liquids, lest the appearance of the stone be irre-
trievably injured. Methylene iodide readily serves
to distinguish the yellow quartz from the true topaz,
with which jewellers often confuse it, the latter stone
sinking in the liquid ; again aquamarine floats, but
the blue topaz, which is often very similar to it,
sinks in methylene iodide.
By saturating methylene iodide with iodine and
iodoform, we have a liquid (d} of density 3'6 ; a
fragment of topaz, 3-55, may be used to indicate
whether the liquid has the requisite density. Un-
fortunately this saturated solution is so dark as to
be almost opaque, and is, moreover, very viscous.
Its principal use is to distinguish diamond, 3'535,
from the brilliant colourless zircon, with which,
apart from a test for hardness, it may easily be
SPECIFIC GRAVITY 69
confused. It is easy to see whether the stone
floats, as it would do if a diamond. To recover a
stone which has sunk, the only course is to pour
off the liquid into another tube, because it is far
too dark for the position of the stone to be seen.
It is possible to employ a similar method for
still denser stones by having recourse to Retgers's
salt, silver-thallium nitrate. This double salt is
solid at ordinary room-temperatures, but has the
remarkable property of melting at a temperature,
75° C., which is well below the point of fusion of
either of its constituents, to a clear, mobile yellow
liquid, which is miscible in any proportion with
water, and has, when pure, a density of 4/6. The
salt may be purchased, or it may be prepared by
mixing 100 grams of thallium nitrate and 64 grams
of silver nitrate, or similar proportions, in a little
water, and heating the whole over a water-bath,
keeping it constantly stirred with a glass rod until
it is liquefied. The two salts must be mixed in the
correct proportions, because otherwise the mixture
might form other double salts, which do not melt
at so low a temperature. A glance at the table of
specific gravities shows that Retgers's salt may be
used for all the gem-stones with the single exception
of zircon (b). There are, however, some objections
to its use. It is expensive, and, unless kept con-
stantly melted, it is not immediately available. It
darkens on exposure to strong sunlight like all
silver salts, stains the skin a peculiar shade of
purple which is with difficulty removed, and in fact
only by abrasion of the skin, and, like all thallium
compounds, is highly poisonous.
It is convenient to have three tubes, fitted as
70 GEM-STONES
before with stoppers or corks, to contain the follow-
ing liquids, when heated : —
(e) Silver-thallium nitrate, reduced to 3'5 ; using
as indicators, peridot or idocrase 3-40 and topaz
3*53.
(/) Silver-thallium nitrate, reduced to 4-0; in-
dicators, topaz 3-53 and sapphire 4-03.
(g) Silver-thallium nitrate, undiluted, 4*6.
The tubes must be heated in some form of water-
bath ; an ordinary glass beaker serves the purpose
satisfactorily. The pure salt should never be
diluted; but the density of the contents of tubes
(e) and (/) may be varied at will, water being
added in order to lower the density, and concentra-
tion by means of evaporation or addition of the
nitrate being employed in order to increase it. To
avoid the discoloration of the skin, rubber finger-
stalls may be used, and the stones should not be
handled until after they have been washed in warm
water. The staining may be minimized if the
hands be well washed in hot water before being
exposed to sunlight. It is advisable to warm the
stone to be tested in a tube containing water be-
forehand lest the sudden heating develop cracks.
A piece of platinum, or, failing that, copper wire is
of service for removing stones from the tubes ; a
glass rod, spoon-shaped at one end, does equally
well. It must be noted that although Retgers's
salt is absolutely harmless to the ordinary gem-
stones — with the exception of opal and turquoise,
which, as has already been stated, being to some
extent porous, should not be immersed in liquids —
it attacks certain substances, for instance, sulphides
and cannot be applied indiscriminately to minerals.
SPECIFIC GRAVITY 71
The procedure described above is intended only
as a suggestion ; the method may be varied to any
extent at will, depending upon the particular re-
quirements. If such tests are made only occasion-
ally, a smaller number of tubes may be used. Thus
one tube may be substituted for the two marked
a and b, the liquid contained in it being diluted as
required, and a series of indicators may be kept
apart in small glass tubes. On the other hand,
any one having constantly to test stones might in-
crease the number of tubes with advantage, and
might find it useful to have at hand fragments of
all the principal species in order to make direct
comparison.
(2) DIRECT WEIGHING
The balance which is necessary in both the
methods described under this head should be
capable of giving results accurate to milligrams,
i.e. the thousandth part of a gram, and con-
sistent with that restriction the beam may be as
short as possible so as to give rapid swings and
thus shorten the time taken in the observations.
A good assay balance answers the purpose
admirably. Of course, it is never necessary to
wait till the balance has come to rest. The mean
of the extreme readings of the pointer attached to
the beam will give the position in which it would
ultimately come to rest. Thus, if the pointer just
touches the eighth division on the right-hand side
and the second on the other, the mean position is
the third division on the right-hand side (|(8 — 2)
= 3). Instead of the ordinary form of chemical
balance, Westphal's form or Joly's spring-balance
72 GEM-STONES
may be employed. Weighings are made more
quickly, but are not so accurate.
In refined physical work the practice known as
double-weighing is employed to obviate any slight
error there may be in the suspension of the balance.
A counterpoise which is heavier than anything to
be weighed is placed in one pan, and weighed.
The counterpoise is retained in its pan throughout
the whole course of the weighings. Any substance
whose weight is to be found is placed in the other
pan, and weights added till the balance swings
truly again. The difference between the two sets
of weights evidently gives the weight of the sub-
stance. Balances, however, are so accurately con-
structed that for testing purposes such refined
precautions are not really necessary.
It is immaterial in what notation the weighings
are made, so long as the same is used throughout,
but the metric system of weights, which is in
universal use in scientific work, should preferably be
employed. Jewellers, however, use carat weights,
and a subdivision to the base 2 instead of decimals,
the fractions being £, £-, £, ^ J& -fa- If these
weights be employed, it will be necessary to convert
these fractions into decimals, and write | = '5oo,
i = -250, i = -i 25, TV = -062, ^= -03 1, ^ = -016.
(a) Hydrostatic Weighing
The principle of this method is very simple.
The stone, the specific gravity of which is required,
is first weighed in air and then when immersed in
water. If W and W be these weights respectively,
then W —W is evidently the weight of the water
SPECIFIC GRAVITY 73
displaced by the stone and having therefore the
same volume as it, and the specific gravity is there-
W
fore equal to w _ w/-
If the method of double-weighing had been
adopted, the formula would be slightly altered.
Thus, suppose that c corresponds to the counter-
poise, w and w' to the stone weighed in air and
water respectively ; then we have W — c — w and
FIG. 33.— Hydrostatic Balance.
W' = c — w'y and therefore the specific gravity is
c - w
equal to — -. .
w - w
Some precautions are necessary in practice to
assure an accurate result A balance intended for
specific gravity work is provided with an auxiliary
pan (Fig. 33), which hangs high enough up to
permit of the stone being suspended underneath.
The weight of anything used for the suspension
must, of course, be determined and subtracted from
the weight found for the stone, both when in air
and when in water. A piece of fine silk is generally
74 GEM-STONES
used for suspending the stone in water, but it should
be avoided, because the water tends to creep up it
and the error thus introduced affects the first place
of decimals in the case of a one-carat stone, the
value being too high. A piece of brass wire shaped
into a cage is much to be preferred. If the same
cage be habitually used, its weight in air and when
immersed in water to the customary extent in such
determinations should be found once for all.
Care must also be taken to remove all air-bubbles
which cling to the stone or the cage ; their presence
would tend to make the value too low. The surface
tension of water which makes it cling to the wire
prevents the balance swinging freely, and renders
it difficult to obtain a weighing correct to a
milligram when the wire dips into water. This
difficulty may be overcome by substituting a liquid
such as toluol, which has a much smaller surface
tension.
As has been stated above, the density of water
at 4° C. is taken as unity, and it is therefore
necessary to multiply the values obtained by the
density of the liquid, whatever it be, at the tempera-
ture of the observation. In Table IX, at the end
of the book, are given the densities of water and
toluol at ordinary room-temperatures. It will be
noticed that a correct reading of the temperature
is far more important in the case of toluol.
Example of a Hydrostatic Determination of
Specific Gravity —
Weight of stone in air = I '47 1 gram
Weight of stone in water = I '067 ,,
SPECIFIC GRAVITY 75
Allowing for the density of water at the tempera-
ture of the room, which was 16° C., the specific
gravity is 3'637. Had no such allowance been
made, the result would have been four units too high
in the third place of decimals. For discriminative
purposes, however, such refinement is unnecessary.
(b) Pycnometer^ or Specific Gravity Bottle
The specific gravity bottle is merely one with
a fairly long neck on which a horizontal mark has
been scratched, and which is closed by a ground
glass stopper. The pycnometer is a refined variety
of the specific gravity bottle. It has two openings :
the larger is intended for the insertion of the stone
and the water, and is closed by a stopper through
which a thermometer passes, while the other,
which is exceedingly narrow, is closed by a stopper
fitting on the outside, and is graduated to facilitate
the determination of the height of the water in it.
The stone is weighed as in the previous method.
The bottle is then weighed, and filled with water
up to the mark and weighed again. The stone is
now introduced into the bottle, and the surplus
water removed with blotting-paper or otherwise
until it is at the same level as before, and the bottle
with its contents is weighed. Let W be the weight
of the stone, w the weight of the bottle, W the
weight of the bottle and the water contained in it,
and W" the weight of the bottle when containing
the stone and the water. Then W -w is the
weight of the water filling the bottle up to the
mark, and W" — w — W is the reduced weight of
water after the stone has been inserted ; the difference,
76 GEM-STONES
W+W- W"t is the weight of the water displaced.
W
The specific gravity is therefore - — — 5.
W + W — W
As in the previous method, this value must be
multiplied by the density of the liquid at the
temperature of the experiment. If the method
of double-weighing be adopted, the formula will be
slightly modified.
Of the above methods, that of heavy liquids, as
it is usually termed, is by far the quickest and the
most convenient for stones of ordinary size, the
specific gravity of which is less than the density of
pure methylene iodide, namely, 3*324, and by its aid
a value may be obtained which is accurate to the
second place of decimals, a result quite sufficient
for a discriminative test. The method is applicable
no matter how small the stone may be, and, indeed,
for very small stones it is the only trustworthy
method ; for large stones it is inconvenient, not only
because of the large quantity of liquid required, but
also on account of the difficulty in estimating with
sufficient certainty the position of the centre of
gravity of the stone. A negative determination may
be of value, especially if attention be paid to the rate
at which the stone falls through the liquid ; the
denser the stone the faster it will sink, but the rate
depends also upon the shape of the stone. Retgers's
salt is less convenient because of the delay involved
in warming it and of the almost inevitable staining
of the hands, but its use presents no difficulty
whatever.
Hydrostatic weighing is always available, unless
the stone be very small, but the necessary weighings
SPECIFIC GRAVITY 77
occupy considerable time, and care must be taken
that no error creeps into the computation, simple
though it be. Even if everything is at hand, a
determination is scarcely possible under a quarter
of an hour.
The third method, which takes even longer, is
intended primarily for powdered substances, and is
not recommended for cut stones, unless there happen
to be a number of tiny ones which are known to
be exactly of the same kind.
The specific gravities of the gem-stones are given
in Table VII at the end of the book.
CHAPTER IX
HARDNESS AND CLE A V ABILITY
EVERY possessor of a diamond ring is aware
that diamond easily scratches window-glass.
If other stones were tried, it would be found that
they also scratched glass, but not so readily, and,
if the experiment were extended, it would be found
that topaz scratches quartz, but is scratched by
corundum, which in its turn yields to the all-
powerful diamond. There is therefore considerable
variation in the capacity of precious stones to
resist abrasion, or, as it is usually termed, in their
hardness. To simplify the mode of expressing this
character the mineralogist Mohs about a century
ago devised the following arbitrary scale, which is
still in general use.
MOHS'S SCALE OF HARDNESS
i. Talc
2. Gypsum
3. Calcite
4. Fluor
5. Apatite
6. Orthoclase
10. Diamond
7. Quartz
8. Topaz
9. Corundum
A finger-nail scratches gypsum and softer sub-
stances. Ordinary window-glass is slightly softer
than orthoclase, and a steel knife is slightly harder ;
HARDNESS AND CLEAV ABILITY 79
a hardened file approaches quartz in hardness, and
easily scratches glass.
By saying that a stone has hardness 7 we merely
mean that it will not scratch quartz, and quartz
will not scratch it. The numbers indicate an order,
and have no quantitative significance whatever. This
is an important point about which mistakes are
often made. We must not, for instance, suppose
that diamond has twice the hardness of apatite.
As a matter of fact, the interval between diamond
and corundum is immensely greater than that
between the latter and talc, the softest of mineral
substances. Intermediate degrees of hardness
are expressed by fractions. The number 8£ for
chrysoberyl means that it scratches topaz as easily
as it itself is scratched by corundum. Pyrope
garnet is slightly harder than quartz, and its
hardness is said therefore to be "j\>
Delicate tests show that the structure of all
crystallized substances is more or less grained, like
that of wood, and the hardness for the same stone
varies in different directions. Kyanite is unique
in this respect, since its hardness ranges from 5 to
7 ; it can therefore be scratched by a knife in some
directions, but not in others. In most substances,
however, the range is so small as to be quite imper-
ceptible. Slight variation is also apparent in the
hardness of different specimens of the same species.
The diamonds from Borneo and New South Wales
are so distinctly harder than those from South
Africa and other localities that, when first discovered,
some difficulty was experienced in cutting them.
Again, lapidaries find that while Ceylon sapphires
are harder than rubies, Kashmir sapphires are softer.
8o GEM-STONES
Hardness is a character of fundamental importance
in a stone intended for ornamental wear, since upon
it depends the durability of the polish and brilliancy.
Ordinary dust is largely composed of grains of
sand, which is quartz in a minute form, and a
gem-stone should therefore be at least as hard as
that Paste imitations are little harder than 5, and
consequently, as experience shows, their polish does
not survive a few weeks' wear. Hardness is,
however, of little use as a discriminative test except
for distinguishing between topaz or harder stone and
paste. Diamond is so much harder than other
stones that it will leave a cut in glass quite different
from the scratch of even corundum. Paste, being so
soft, readily yields to the file, and is thus easily
distinguished from genuine stones. In applying the
test to a cut stone, it is best to remove it from its
mount and try the effect on the girdle, because
any scratch would be concealed afterwards by the
setting. Any mark should be rubbed with the
finger to assure that it is not due to powder from
the scratching agent ; confusion may often be caused
in this way when the two substances are of nearly
the same hardness.
The degrees of hardness of the gem-stones are
given in Table VIII at the end of the book.
It must not be overlooked that extreme hardness
is compatible with cleavability in certain directions
intimately connected with the crystalline structure ;
the property, in fact, characterizes many mineral
species of different degrees of hardness. Diamond
can be split in four directions parallel to the faces of
the regular octahedron, a property utilized by the
HARDNESS AND CLEAVABILITY 81
lapidary for shaping a stone previous to cutting it.
Topaz cleaves with considerable ease at right angles
to the principal crystallographic axis. Felspar has
two directions of cleavage nearly at right angles to
one another. The new gem-stone, kunzite, needs
cautious handling owing to the facility with which
it splits in two directions mutually inclined at
about 70°.
All stones are more or less brittle, and will be
fractured by a sufficiently violent blow, but the
irregular surface of a fracture cannot be mistaken
for the brilliant flat surface given by a cleavage.
The cleavage is by no means induced with equal
facility in the species mentioned above. A consider-
able effort is required to split diamond, but in the
case of topaz or kunzite incipient cleavage in the
shape of flaws may be started if the stone be merely
dropped on to a hard floor.
CHAPTER X
ELECTRICAL CHARACTERS
THE definite orientation of the molecular
arrangement of crystallized substances leads
in many cases to attributes which vary with the
direction and are revealed by the electrical properties.
If a tourmaline crystal be heated in a gas or alcohol
flame it becomes charged with electricity, and, since
it is at the same time a bad conductor, static charges
of opposite sign appear at the two ends. Topaz
shows similar characters, but in a lesser degree.
Quartz, if treated in the same way, shows charges
of opposite sign on different sides, but the
phenomenon may be masked by intimate twinning
and consequent overlapping of the contrary areas.
The phenomenon may also be seen when the
stones are cut. The most convenient method for
detecting the existence of the electrical charges is
that devised by Kundt A powder consisting of a
mixture of red lead and sulphur is placed in a
bellows arrangement and blown through a sieve
at one end on to the stone. Owing to the friction
the particles become electrified — red lead positively
and sulphur negatively — and are attracted by the
charges of opposing sign, which will therefore be
betrayed by the colour of the dust at the corre-
sponding spot. The powder must be kept dry ;
ELECTRICAL CHARACTERS 83
otherwise a chemical reaction may occur leading to
the formation of lead sulphide, recognizable by its
black colour. Bucker has suggested as an alterna-
tive the use of sulphur, coloured red with carmine,
the negative element, and yellow lycopodium, the
positive element.
Diamond, topaz, and tourmaline are powerful
enough, when electrified by friction with a cloth, to
attract fragments of paper, the electrification being
positive. Amber develops considerable negative
electricity when treated in a similar manner.
Diamond is translucent to the Rontgen (X) rays ;
glass, on the other hand, is opaque to them, and
this test distinguishes brilliants from paste imitations.
Diamond also, unlike glass, phosphoresces under
the influence of radium, a property characterizing
also kunzite.
It will be seen that the electrical characters,
although of considerable interest to the student,
are, on account of their limited application and
difficulty of test, of little service for the discrimination
of gem-stones.
PART I— SECTION B
THE TECHNOLOGY OF GEM-
STONES
CHAPTER XI
UNIT OF WEIGHT
THE system in use for recording the weights of
precious stones is peculiar to jewellery.
The unit, which is known as the carat, bears no
simple relation to any unit that has existed among
European nations, and indubitably has been intro-
duced from the East. When man in early days
sought to record the weights of small objects, he
made use of the most convenient seeds or grains
which were easily obtainable and were at the same
time nearly uniform in size. In Europe the
smallest unit of weight was the barley grain.
Similarly in the East the seeds of some leguminous
tree were selected. Those of the locust-tree, Cera-
tonia siliqua, which is common in the countries
bordering the Mediterranean, on the average weigh
so nearly a carat that they almost certainly formed
the original unit. It is, indeed, from the Greek
Kepdnov, little horn, which refers to the shape of the
pods, that the word carat is derived.
UNIT OF WEIGHT 85
It is one of the eccentricities of the jewellery
trade that precision should not have been given to
the unit of weight. Not only does it vary at most
of the trade centres in the world, but it is not even
always constant at each centre. The difference
is negligible in the case of single stones of
ordinary size, but becomes a matter of serious
importance when large stones, or parcels of small
stones, are bought and sold, particularly when the
stones are very costly. Attempts have been made
at various times to secure a uniform standard, but
as yet with only partial success. In 1871 the carat
defined as the equivalent of 0*20500 gram was
suggested at a meeting of the principal jewellers
of Paris and London, and was eventually accepted
in Paris, New York, Leipzig, and Borneo. It has,
however, recently been recognized that in view of
the gradual spread of the metric system of weights
and measures the most satisfactory unit is the
metric carat of one-fifth (0*2) gram. This has now
been constituted the legal carat of France and
Belgium, and no doubt other countries will follow
their example. The carat weight obtaining in
London weighs about 0-20530 gram, and the
approximate equivalents in the gram at other
centres are as follows: — Florence 0-19720, Madrid
0*20539, Berlin 0*20544, Amsterdam 0*20570,
Lisbon 0*20575, Frankfort - on - Main 0*20577,
Vienna 0-20613, Venice 0*20700, and Madras
0*20735. The gram itself is inconveniently large
to serve as a unit for the generality of stones met
with in ordinary jewellery.
The notation for expressing the sub-multiples
of the carat forms another curious eccentricity.
86 GEM-STONES
Fractions are used which are powers of the half:
thus the half, the half of that, i.e. the quarter, and
so on down to the sixty-fourth, and the weight of
a stone is expressed by a series of fractions, e.g.
SaieV carats. In the case of diamond a single
unreduced fraction to the base 64 is substituted
in place of the series of single fractions, and the
weight of a stone is stated thus, 4|-£ carats. With
the introduction of the metric carat the more con-
venient and rational decimal notation would, of
course, be simultaneously adopted.
Figs. 34-39 illustrate the exact sizes of diamonds
10 carats.
FIGS. 34-39.— Exact Sizes of Brilliants of various Weights.
of certain weights, when cut as brilliants. The
sizes of other stones depends upon their specific
gravity, the weight varying as the volume multiplied
by the specific gravity. Quartz, for instance, has
a low specific gravity and would be perceptibly
larger, weight for weight; zircon, on the other
hand, would be smaller.
It has been found more convenient to select
a smaller unit in the case of pearls, namely, the
pearl-grain, four of which go to the carat.
Stencil gauges are in use for measuring approxi-
mately the weight in carats of diamond brilliants and
of pearls, which in both instances must be unmounted.
A more accurate method for determining the weight
UNIT OF WEIGHT 87
of diamonds has been devised by Charles Moe, which
is applicable to either unmounted or mounted stones.
By means of callipers, which read to three-tenths of
a millimetre, the diameter and the depth of the stone
are measured, and by reference to a table the corre-
sponding weight is found ; allowance is made for
the varying fineness of the girdle, and, in the case of
large stones, for the variation from a strictly circular
section.
Since this chapter was written the movement in
favour of the metric carat has made rapid progress,
and this unit will soon have been adopted as the
legal standard all over the world, even in countries,
such as the British Isles and the United States, where
the metric system is not in use. The advantage of
an international unit is too obvious to need arguing.
CHAPTER XII
FASHIONING OF GEM-STONES
ALTHOUGH many of the gem-stones have
been endowed by nature with brilliant
lustrous faces and display scintillating reflections
from their surfaces, yet their form is never such as
to reveal to full perfection the optical qualities upon
which their charm depends. Moreover, the natural
faces are seldom perfect ; as a rule the stones are
broken either through some convulsion of the earth's
crust or in course of extraction from the matrix in
which they have lain, or they are roughened by
attrition against matter of greater hardness, or worn
by the prolonged action of water, or etched by
solvents. Beautiful octahedra of diamond or spinel
have been mounted without further embellishment,
but even their appearance might have been much
improved at the lapidary's hands.
By far the oldest of the existing styles of cutting
is the rounded shape known as cabochon, a French
word derived from the Latin cabo, a head. In the
days of the Roman Empire the softer stones were
often treated in this manner ; such stones were
supposed to be beneficial to those suffering from
short-sightedness, the reason no doubt being that
transparent stones when cut as a double cabochon
formed a convex lens. According to Pliny, Nero
JEWELLERY DESIGNS
FASHIONING OF GEM-STONES 89
had an emerald thus cut, through which he was
accustomed to view the gladiatorial shows. This
style of cutting was long a favourite for coloured
stones, such as emerald, ruby, sapphire, and garnet,
but has been abandoned in modern practice except
for opaque, semi-opaque, and imperfect stones.
The crimson garnet, which was at one time known
by the name carbuncle, was so systematically thus cut
that the word has come to signify a red garnet of
this form. It was a popular brooch-stone with our
grandmothers, but is no longer in vogue. The East
still retains a taste for stones cut in the form of beads
and drilled through the centre; the beads are
threaded together, and worn as
necklaces. The native lapidaries
often improve the colour of pale
emeralds by lining the hole with
. / FIG. 40.— Double
green paint. (Convex) Ca.
The cabochon form may be of bochon.
three different kinds. In the first,
the double cabochon (Fig. 40), both the upper and
the under sides of the stones are curved. The
curvature, however, need not be the same in each
case; indeed, it is usually markedly different
Moonstones and starstones are generally cut very
steep above and shallow underneath. Occasion-
ally a ruby or a sapphire is, when cut in this
way, set with the shallow side above, because the
light that has penetrated into the stone from above
is more wholly reflected from a steep surface with
consequent increase in the glow of colour from the
stone. Opals are always cut higher on the exposed
side, but the slope of the surface varies considerably ;
they are generally cut steeply when required for
90 GEM-STONES
mounting in rings. Chrysoberyl cat's-eyes are
invariably cut with curved bases in order to preserve
the weight as great as possible. The double
cabochon form with a shallow surface underneath
merges into the second kind (Fig. 41) in which the
under side is plane, the form commonly employed
for quartz cat's-eyes, and occasion-
ally also for carbuncles. In this type
the plane side is invariably mounted
FIG. 41. — Simple r J
Cabochon. down wards. In the third form
(Fig. 42) the curvature of the under
surface is reversed, and the stone is hollowed out
into a concave shape. This style is reserved for
dark stones, such as carbuncles, which, if cut at
all thick, would show very little colour. A piece
of foil is often placed in the hollow in order to
increase the reflection of light, and
thus to heighten the colour effect. .^^SS^^
In early days it was supposed that
, , , f ,. FIG. 42.— Double
the extreme hardness of diamond (Concavo - con-
precluded the possibility of fashion- vex) Cabochon.
ing it, and up to the fifteenth century
all that was done was to remove the gum-like skin
which disfigured the Indian stones and to polish the
natural facets. The first notable advance was
made in 1475, when Louis de Berquem discovered,
as it is said quite by accident, that two diamonds if
rubbed together ground each other. With confident
courage he essayed the new art upon three large
stones entrusted to him by Charles the Bold, to the
entire satisfaction of his patron. The use of wheels
or discs charged with diamond dust soon followed,
but at first the lapidaries evinced their victory over
such stubborn material by grinding diamond into
FASHIONING OF GEM-STONES 91
divers fantastic shapes, and failed to realize how
much might be done to enhance the intrinsic beauty
of the stones by the means now at their disposal
The Indian lapidaries arrived at the same discovery
independently, and Tavemier found, when visiting
the country in 1665, a large number of diamond
cutters actively employed. If the
stone were perfectly clear, they con-
tented themselves with polishing the
natural facets ; but if it contained
flaws or specks, they covered it with
numerous small facets haphazardly
placed. The stone was invariably
left in almost its original shape,
and no effort was made to improve the symmetry.
For a long time little further progress wasmade,
and even nearly a century after Berquem the only
regular patterns known to Kentmann, who wrote in
1 562, were the diamond-point and the diamond-table
(Figs. 43—44). The former consisted of the natural
octahedron facets ground to regular
/ \ shape, and was long employed for
~~x the minute stones which were set
in conjunction with large coloured
FIG —Table s^ones *n rings. The table repre-
Cut (side view), sented considerably greater labour
One corner of the regular octahedron
was ground down until the artificial facet thus pro-
duced was half the width of the stone, while the
opposite corner was slightly ground.
Still another century elapsed before the introduc-
tion of the rose pattern, which comprised twenty-
four triangular facets and a flat base (Figs. 45—46),
the stone being nearly hemispherical in shape. This
92 GEM-STONES
style is said to have been the invention of Cardinal
Mazarin, but probably he was the first to have
diamonds of any considerable size cut in this form.
At the present day only tiny stones are cut as
roses.
A few more years passed away, and at length at
the close of the seventeenth century diamond came
by its own when Vincenzio Peruzzi,
a Venetian, introduced the brilliant
form of cutting, and revealed for the
first time its amazing c fire.' Except
for minor changes this form remains
,FiG.45.-RoseCut tO this day the standard style for
(top view). the shape of diamond, and the word
brilliant is commonly employed to
denote diamond cut in this way. So obviously and
markedly superior is the style to all others that
upon its discovery the owners of large roses had
them re-cut as brilliants despite the loss in weight
necessitated by the change.
The brilliant form is derived from the old table
by increasing the number of facets
and slightly altering the angles
pertaining to the natural octa-
hedron. In a perfect brilliant FlG 46<_Rose Cut
(Figs. 47-49) there are altogether '(side view).
58 facets, 33 above and 25 below
the girdle, as the edge separating the upper and
lower portions of the stone is termed, which are
arranged in the following manner. Eight star-
facets, triangular in shape, immediately surround
the large table-facet. Next come four large
templets or bezels, quadrilateral in form, arranged
in pairs on opposite sides of the table-facet, the
FASHIONING OF GEM-STONES
93
four quoins or lozenges, similar in shape, coming
intermediately between them ; in modern practice,
however, these two sets are identical in shape and
size, and there are consequently eight facets of the
same kind instead of two sets of four. The eight
FlG. 47.— Brilliant
Cut (top view).
FIG. 48.— Brilliant
Cut (base view).
cross or skew facets and the eight skill facets, in
both sets the shape being triangular, form the
boundary of the girdle ; modern brilliants usually have
instead sixteen facets of the same shape and size.
The above 33 facets lie above the girdle and form
the crown of the stone. Imme-
diately opposite and parallel to
the table is the tiny culet. Next
to the latter come the four large
pavilion facets with the four quoins
intermediately between them, both
sets being five-sided but nearly
quadrilateral in shape ; these again are usually com-
bined into eight facets of the same size. Eight cross
facets and eight skill facets, both sets, like those in
the crown, being triangular in shape, form the lower
side of the girdle ; these also are generally united
into a set of sixteen similar facets. These 2 5 facets
which lie below the girdle comprise the ' pavilion,'
FIG. 49.— Brilliant
Cut (side view).
94 GEM-STONES
or base of the stone. In a regular stone properly
cut a templet is nearly parallel to a pavilion, and
an upper to a lower cross facet. The contour of the
girdle is usually circular, but occasionally assumes
less symmetrical shapes, as for instance in drop-
stones or pendeloques, and the facets are at the
same time distorted. The number of facets may
with advantage be increased in the case of large
stones. An additional set of eight star facets is
often placed round the culet, the total number then
being 66. It may be mentioned that the largest
stone cut from the Cullinan has the exceptional
number of 74 facets.
In order to secure the finest optical effect certain
proportions have been found necessary. The depth
of the crown must be one-half that of the base, and
therefore one-third the total depth of the stone, and
the width of the table must be slightly less than
half that of the stone. The culet should be quite
small, not more in width than one-sixth of the
table ; it is, in fact, not required at all except to
avoid the danger of the point splintering. The
girdle should be as thin as is compatible with
strength sufficient to prevent chipping in the process
of mounting the stone ; if it were left thick, the
rough edge would be visible by reflection at the
lower facets, and would, especially if at all dirty,
seriously affect the quality of the stone. The shape
of the stone is largely determined by the sizes of
the templets in the crown and the pavilions in the
base as compared with that of the table, or, what
comes to the same thing, by the inclinations at
which they are cut to that facet. If the table
had actually half the width of the stone, the
FASHIONING OF GEM-STONES 95
angle l between it and a templet would be exactly
half a right angle or 45°; it is, however, made
somewhat smaller, namely, about 40°. A pavilion,
being parallel to a templet, makes a similar angle
with the culet. The cross facets are more
steeply inclined, and make an angle of about 45°
with the table or the culet, as the case may be.
The star facets, on the other hand, slant per-
ceptibly less, and make an angle of only about 26°
with the table. A latitude of some 4° or 5° is
possible without seriously affecting the ' fire ' of the
stone.
The object of the disposition of the facets on a
brilliant is to assure that all the light that enters
the stone, principally by way of the table, is wholly
reflected from the base and emerges through the
crown, preferably by way of the inclined facets. A
brilliant-cut diamond, if viewed with the table between
the observer and the light, appears quite dark except
for the small amount of light escaping through the
culet. Light should therefore fall on the lower facets
at angles greater than the critical angle of total-
reflection, which for diamond is 24° 26'. The
pavilions should be inclined properly at double this
angle, or 48° 52', to the culet; but a ray that
emerges at a pavilion in the actual arrangement
entered the table at nearly grazing incidence, and
the amount of light entering this facet at such acute
perspective is negligible. On the other hand,
after reflection at the base light must, in order to
emerge, fall on the crown at less than the critical angle
1 In accordance with the usual custom the angle between the facets
is taken as that between their normals, or the supplement of the salient
angle.
96
GEM-STONES
of total-reflection. In Fig. 50 are shown diagram
matically the paths of rays that entered the table
in divers ways. The ray emerging again at the
table suffers little or no dispersion and is almost
white, but those coming out through the inclined
facets are split up into the rainbow effect, known as
'fire,' for which diamond is so famous. It is in
order that so much of the light entering by the
FlG. 50. — Course of the Rays of Light passing through a Brilliant.
table may emerge through the inclined facets of
the crown that the pavilions are inclined at not
much more than 40° to the culet. It might be
suggested that instead of being faceted the stone
should be conically shaped, truncated above and
nearly complete below. The result would no doubt
be steadier, but, on the other hand, far less pleasing
It is the ever-changing nuance that chiefly attracts
the eye ; now a brilliant flash of purest white, anon
FASHIONING OP GEM-STONES 9?
a gleam of cerulean blue, waxing to richest orange
and dying in a crimson glow, all intermingled with
the manifold glitter from the surface of the stone.
Absolute cleanliness is essential if the full beauty of
any stone is to be realized, but this is particularly
true of diamond. If the back of the stone be
clogged with grease and dirt, as so often happens
in claw-set rings, light is no longer wholly reflected
from the base ; much of it escapes, and the amount
of ' fire ' is seriously diminished.
Needless to state, lapidaries make no careful
angular measurements when cutting stones, but judge
of the position of the facets entirely by eye. It
sometimes therefore happens that the permissible
limits are overstepped, in which event the stone is
dead and may resist all efforts to vivify it short of
the heroic course of re-cutting it, too expensive a
treatment in the case of small stones.
The factors that govern the properties of a
brilliant-cut stone are large colour-dispersion, high
refraction, and freedom from any trace of intrinsic
colour. The only gem-stone that can vie with
diamond in these respects is zircon. Although it is
rare to find a zircon naturally without colour, yet
many kinds are easily deprived of their tint by the
application of heat. A brilliant-cut zircon is, indeed,
far from readily distinguished by eye from diamond,
and has probably often passed as one, but it may
easily be identified by its large double refraction
(cf. p. 41) and inferior hardness. The remaining
colourless stones, such as white sapphire, topaz, and
quartz (rock-crystal), have insufficient refractivity to
give total-reflection at the base, and, moreover, they
are comparatively deficient in ' fire.'
7
98 GEM-STONES
A popular style of cutting which is much in
vogue for coloured stones is the step- or trap-cut,
consisting of a table and a series of facets with
parallel horizontal edges (Figs. 51—52) above and
below the girdle ; in recent jewellery, however, the top
of the stone is often brilliant-cut The contour may
be oblong, square, lozenge, or heart-shaped, or have
less regular forms. The table is
sometimes slightly rounded. Since
the object of this style is primarily
to display the intrinsic colour of
view). brilliant play of light from the
interior, no attempt is made to
secure total-reflection at the lower facets. The
stone therefore varies in depth according to its
tint ; if dark, it is cut shallow, lest light be wholly
absorbed within, and the stone appear practically
opaque, but if light, it is cut deep, in order to
secure fullness of tint. Much precision in shape
and disposition of the facets is
not demanded, and the stones are
usually cut in such a way that,
provided the desired effect is ob- FlG;, S2.-SteP- or
r . Trap -Cut (side
tamed, the weight is kept as great view).
as possible; we may recall that
stones are sold by weight In considering what
will be the optical effect of any particular shape,
regard must be had to the effective colour of the
transmitted light. For instance, although sapphire
and ruby belong to the same species and have
the same refractive indices, yet, since the former
transmits mainly blue and the latter red light, they
have for practical purposes appreciably different
FASHIONING OF G&M-STONES 99
indices, and lapidaries find it therefore possible to
cut the base of ruby thicker than that of sapphire,
and thus keep the weight greater. It is instructive
too what can be done with the most unpromising
material by the exercise of a little ingenuity.
Thus Ceylon sapphires are often so irregularly
coloured that considerable skill is called for in
cutting them. A stone may, for instance, be
almost colourless except for a single spot of blue ;
yet, if the stone be cut steeply and the spot be
brought to the base, the effect will be precisely the
same as if the stone were uniformly coloured,
because all the light emerging from the stone has
passed through the spot at the base and therefore
been tinted blue.
The mechanism employed in the fashioning of
gem-stones is simple in character, and comprises
merely metal plates or wheels for slitting, and discs
or laps for grinding and polishing the stones, the
former being set vertically and rotated about
horizontal spindles, and the latter set horizontally
and rotated about vertical spindles. Mechanical
power is occasionally used for driving both kinds
of apparatus, but generally, especially in slitting
and in delicate work, hand-power is preferred. In
the East native lapidaries make use of vertical wheels
(Plate XIII) also for grinding and polishing stones,
which explains why native-cut stones never have
truly plane facets ; it will be noticed from the
picture that a long bow is used to drive the
spindle.
Owing to the unique hardness of diamond it can
be fashioned only by the aid of its own powder.
The process differs therefore materially from the
ioo GEM-STONES
cutting of the remaining gem-stones, and will be
described separately. Indeed, so different are the
two classes of work that firms seldom habitually
undertake both.
The discovery of the excellent cleavage of
diamond enormously reduced the labour of cutting
large stones. A stone containing a bad flaw may
be split to convenient shape in as many minutes
as the days or even weeks required to grind it down.
The improvement in the appliances and the provision
of ample mechanical power has further accelerated
the process and reduced the cost. Two years were
occupied in cutting the diamond known as the Pitt
or Regent, whereas in only six months the colossal
Cullinan was shaped into two large and over a
hundred smaller stones with far less loss of material.
Although the brilliant form was derived from the
regular octahedron, it by no means follows that,
because diamond can be cleaved to the latter form,
such is the initial step in fashioning the rough mass.
The aim of the lapidary is to cut the largest possible
stone from the given piece of rough, and the finished
brilliant usually bears no relation whatever to the
natural octahedron. The cleavage is utilized only
to free the rough of an awkward and useless excres-
cence, or of flaws. Although the octahedron is one
of the common forms in which diamond is found, it
is rarely regular, and oftener than not one of the
larger faces is made the table.
The old method, which is still in use, for roughly
fashioning diamonds is that known as bruting, from
the French word, bmtage, for the process, or as
shaping. Two stones of about the same size are
selected, and are firmly attached by means of a hard
FASHIONING OF GEM-STONES 101
cement to the ends of two holders, which are held one
in each hand, and rubbed hard, one against the other,
until surfaces of the requisite size are developed on
each stone. During the process the stones are held
over a small box, which catches the precious powder.
A fine sieve at the bottom of the box allows the
powder to fall through into a tray underneath, but
holds back anything larger. By means of two vertical
pins placed one on each side of the box the holders
are retained more easily in the desired position, and
the work is thrown mainly on the thumbs. This
work continued day after day has a very disfiguring
effect upon the hands despite the thick gloves that
are worn to protect them ; the skin of the thumbs
grows hard and horny, and the first and second
fingers become swollen and distorted. When the
surfaces have thus been formed, the stone is handed
to the polisher, who works them into the correct
shape and afterwards polishes them, the stone
passing backwards and forwards several times
between the cutter and the polisher. The table,
four templets, culet and four pavilions are first
formed and polished, so that the table has a square
shape. Next the quoins are developed and polished,
and finally the small facets are polished on, not
being shaped first. In modern practice the process
of bruting has been modified in some cases by the
introduction of machinery, and the facets are ground
on, with considerable improvement in the regularity
of their size and disposition, and reduction in the
amount of polishing required. Moreover, to obviate
the loss of material resulting from continued grinding,
large stones are first sliced by means of rapidly-re-
volving copper wheels charged with diamond powder.
102 GEM-STONES
The laps used for polishing diamonds are made
of a particular kind of soft iron, which is found to
surpass any other metal in retaining the diamond
powder. They are rotated at a high rate of speed,
which is about 2000 to 2500 revolutions a minute,
and the heat developed by the friction at this speed
is too great for a cement to be used ; a solder or
fusible alloy, composed of one part tin to three
parts lead, therefore takes its place. The solder
is held in a hollow cup of brass which is from
its shape called a ' dop,' an old Dutch word meaning
shell. Its external diameter is ordinarily about i|
in. (4 cm.), but larger dops are, of course, used
for large stones. A stout copper stalk is attached
to the bottom of the dop ; it is visible in the view
of the dop shown at e on Plate VI, and two slabs of
solder are seen lying in front of the dop. The dop
containing the solder is placed in the midst of a
non-luminous flame and heated until the solder
softens, when it is removed by means of the small
tongs, c, and placed upright on a stand such as
that shown at a. The long tongs, d, are used for
shaping the solder into a cone at the apex of which
the diamond is placed. The solder is worked well
over the stone so that only the part to undergo
polishing is exposed. A diamond in position is
shown at/. The top of the stand is saucer-shaped
to catch the stone should it accidentally fall off the
dop, and to prevent pieces of solder falling on the
hand. While still hot, the dop with the diamond in
position on the solder is plunged into cold water in
order to cool it. The fact that the stone withstands
this drastic treatment is eloquent testimony to its
good thermal conductivity ; other gem-stones would
POLISHING DIAMONDS
FASHIONING OF GEM-STONES 103
promptly split into fragments. It may be remarked
that so high is the temperature at which diamond
burns that it may be placed in the gas flame without
any fear of untoward results. The dop is now ready
for attachment to an arm such as that shown at b ;
the stalk of the dop is placed in a groove running
across the split end of the arm, and is gripped tight
by means of a screw worked by the nut which is
visible in the picture.
Four such arms, each with a dop, are used with
the polishing lap (Plate VII), and each stands on
two square legs on the bench. Pins, /, in pairs
are fixed to the bench to prevent the arms being
carried round by the friction ; one near the lap holds
the arm not far from the dop, and the other engages
in a strong metal tongue, which is best seen at the
end of the arm b on Plate VI. Though the arm,
which is made of iron, is heavy, yet for polishing
purposes it is insufficient, and additional lead weights
are laid on the top of it, as in the case of the arm at
the back on Plate VII. The copper stalk is strong,
yet flexible, and can be bent to suit the position of
the facet to be polished; on Plate VII the dops a
and b are upright, but the other two are inclined.
In addition to the powder resulting from bruting,
boart, i.e. diamonds useless for cutting, are crushed
up to supply polishing material, and a little olive oil
is used as a lubricant. Owing to the friction so
much heat is developed that even the solder would
soften after a time, and therefore, as a precaution,
the dop is from time to time cooled by immersion
in water. The stone has constantly to be re-set,
about six being the maximum even of the tiny
facets near the girdle that can be dealt with by
1 04 GEM-STONES
varying the inclination of the dop. As the work
approaches completion the stone is frequently in-
spected, lest the polishing be carried too far for the
development of the proper amount of ' fire.' When
finished, the stones are boiled in sulphuric acid to
remove all traces of oil and dirt.
The whole operation is evidently rough and ready
in the extreme ; but such amazing skill do the
lapidaries acquire, that even the most careful in-
spection by eye alone would scarce detect any want
of proper symmetry in a well-cut stone.
The fashioning of coloured stones, as all the
gem-stones apart from diamond are termed in the
jewellery trade, is on account of their inferior
hardness a far less tedious operation. They are
easily slit, for which purpose a vertical wheel
(Plate VIII) made of soft iron is used ; it is charged
with diamond dust and lubricated with oil, generally,
paraffin. When slit to the desired size, the stone is
attached to a conveniently shaped holder by means
of a cement, the consistency of which varies with
the hardness of the stone. It is set in the cement
in such a way that the plane desired for the table
facet is at right angles to the length of the holder,
and the whole of the upper part or crown is finished
before the stone, is removed from the cement. The
lower half or base is treated in a similar manner.
Thus in the process of grinding and polishing the
stone is only once re-set ; as was stated above,
diamond demands very different treatment. Again,
all coloured stones are ground down without any
intermediate operation corresponding to bruting.
The holder is merely held in the hand, but to
maintain its position more exactly its other end,
PLATE VIII
ING COLOURED STi'NKS
VCETING MACHI
FASHIONING OF GEM-STONES 105
which is pointed, is inserted in one of the holes that
are pierced at intervals in a vertical spindle placed
at a convenient distance from the lap (Plate VIII),
which one depending upon the inclination of the
facet to be formed. For hard stones, such as ruby
and sapphire, diamond powder is generally used as
the abrasive agent, while for the softer stones emery,
the impure corundum, is selected ; in recent years
the artificially prepared carborundum, silicide of
carbon corresponding to the formula CSi, which is
harder than corundum, has come into vogue for
grinding purposes, but it is unfortunately useless
for slitting, because it refuses to cling to the wheel.
To efface the scratches left by the abrasive agent
and to impart a brilliant polish to the facets,
material of less hardness, such as putty-powder,
pumice, or rouge, is employed ; in all cases the
lubricant is water. The grinding laps are made
of copper, gun-metal, or lead ; and pewter or wooden
laps, the latter sometimes faced with cloth or
leather, are used for polishing. As a general
rule, the harder the stone the greater the speed of
the lap.
As in the case of diamond, the lapidary judges of
the position of the facet entirely by eye and touch,
but a skilled workman can develop a facet very
close to the theoretical position. During recent years
various devices have been invented to enable him to
do his work with greater facility. A machine of
this kind is illustrated on Plate IX. The stone is
attached by means of cement to the blunt end, d, of
the holder, b, which is of the customary kind, while
the other end is inserted in a hole in a wooden
piece, a} which is adjustable in height by means of
io6 GEM-STONES
the screw above it. The azimuthal positions of the
facets are arranged by means of the octagonal collar,
c, the sides of which are held successively in turn
against the guide, e. The stand itself is clamped
to the bench. The machine is, however, little
used except for cheap stones, because it is too
accurate and leads to waste of material. Stones are
sold by weight, and so long as the eye is satisfied,
no attempt is made to attain to absolute symmetry
of shape.
The pictures on Plates X-XIII illustrate lapidaries'
workshops in various parts of the world. The first
two show an office and a workshop situated in
Hatton Garden, London ; in the former certain of
the staff are selecting from the parcels stones suit-
able for cutting. The third depicts a more primitive
establishment at Ekaterinburg in the Urals. The
fourth shows a typical French family — pere, mere, et
fits — in the Jura district, all busily engaged ; on
the table will be noticed a faceting machine of the
kind described above. In the fifth picture a native
lapidary in Calcutta is seen at work with the driving
bow in his right, and the stone in his left, hand.
A curious difference exists in the systems of
charging for cutting diamonds and coloured stones.
The cost of cutting the latter is reckoned by the
weight of the finished stone, the rate varying from
is. to 8s. a carat according to the character of the
stone and the difficulty of the work ; while in the
case of diamonds, on the other hand, the weight of
the rough material determines the cost, the rate
being about los. to 403. a carat according to the
size, which on the average is equivalent to about
303. to 1 2 os. a carat calculated on the weight
FASHIONING OF GEM-STONES 107
of the finished stone. The reason of the distinction
is obviously because the proper proportions in
a brilliant-cut diamond must be maintained,
whatever be the loss in weight involved ; in
coloured stones the shape is not of such primary
importance.
When finished, the stone finds its way with
others akin to it to the manufacturing jeweller's
establishment, where it is handed to the setter, who
mounts it in a ring, necklace, brooch, or whatever
article of jewellery it is intended for. The metal
used in the groundwork of the setting is generally
gold, but platinum is also employed where an
unobtrusive and untarnishable metal is demanded,
and silver finds a place in cheaper jewellery, although
it is seriously handicapped by its susceptibility to
the blackening influence of the sulphurous fumes
present in the smoke-laden atmosphere of towns.
The stone may be either embedded in the metal
or held by claws. The former is by far the
safer, but the latter the more elegant, and it has the
advantage of exposing the stone d jour, to use the
French jewellers' expression, so that its genuineness
is more evidently testified. It is very important that
the claw setting be periodically examined, lest the
owner one day experience the mortification of finding
that a valuable stone has dropped out ; gold, owing
to its softness, wears away in course of time.
Up to quite recent years modern jewellery was
justly open to the criticism that it was lacking in
variety, that little attempt was made to secure
harmonious association in either the colour or the
lustre of the gem-stones, and that the glitter of the
gold mount was frequently far too obtrusive. Gold
1 08 GEM-STONES
consorts admirably with the rich glow of ruby, but is
quite unsuited to the gleaming fire of a brilliant.
Where the metal is present merely for the mechanical
purpose of holding the stones in position, it should
be made as little noticeable as possible. The artistic
treatment of jewellery is, however, receiving now
adequate attention in the best Paris and London
houses. Some recent designs are illustrated on
Plates IV and V.
PLA TE XIII
CHAPTER XIII
NOMENCLATURE OF PRECIOUS STONES
THE names in popular use for the principal
gem-stones may be traced back to very early
times, and, since they were applied long before the
determinative study of minerals had become a
science, their significance has varied at different
dates, and is even now far from precise. No
ambiguity or confusion could arise if jewellers
made use of the scientific names for the species,
but most of them are unknown or at least
unfamiliar to those unversed in mineralogy, and to
banish old-established names is undesirable, even if
the task were not hopeless. The name selected for
a gem-stone may have a very important bearing on
its fortunes. When the love-sick Juliet queried
' What's in a name ? ' her mind was wandering far
from jewels ; for them a name is everything. The
beautiful red stones that accompany the diamond in
South Africa were almost a drug in the market
under their proper title — garnet, but command a
ready sale under the misnomer ' Cape-ruby.' To
many minds there is a subtle satisfaction in the
possession of a stone which is assumed to be a
sort of ruby that would be destroyed by the know-
ledge that the stone really belonged to the Cinderella
species of gem-stones — the despised garnet. For
no GEM-STONES
similar reasons it was deemed advisable to offer the
lustrous green garnet found some thirty and odd
years ago in the Ural Mountains as ' olivine/ not a
happy choice since their colour is grass- rather than
olive-green, apart from the fact that the term is in
general use in science for the species known in
jewellery as peridot.
The names employed in jewellery are largely
based upon the colour, the least reliable from a
determinative point of view of all the physical
characters of gem-stones. Qualifying terms are
employed to distinguish stones of obviously different
hardness. ' Oriental ' distinguishes varieties of
corundum, but does not imply that they necessarily
came from the East ; the finest gem-stones originally
reached Europe by that road, and the hardest
coloured stones consequently received that term of
distinction.
Nearly all red stones are grouped under the
name ruby, which is derived from a Latin word,
ruber, meaning red, or under other names adapted
from it, such as rubellite, rubicelle. It is properly
applied to red corundum ; ' balas ' ruby is spinel,
which is associated with the true ruby at the Burma
mines and is similar in appearance to it when cut,
and ' Cape ' ruby, is, as has been stated above, a
garnet from South Africa. Rubellite is the lovely
rose-pink tourmaline, fine examples of which have
recently been discovered in California, and rubicelle
is a less pronouncedly red spinel. Sapphire is by
far the oldest and one of the most interesting of the
words used in the language of jewels. It occurs in
Hebrew and Persian, ancient tongues, and means
blue. It was apparently employed for lapis lazuli
NOMENCLATURE OF PRECIOUS STONES 1 1 1
or similar substance, but was transferred to the blue
corundum upon the discovery of this splendid stone.
Oblivious of the real meaning of the word, jewellers
apply it in a quasi-generic sense to all the varieties
of corundum with the exception of the red ruby, and
give vent to such incongruous expressions as ' white
sapphire,' ' yellow sapphire ' ; it is true such stones
often contain traces of blue colour, but that is not
the reason of the terms. ' Brazilian ' sapphire is
blue tourmaline, a somewhat rare tint for this species.
The curious history of the word topaz will be found
below in the chapter dealing with the species of that
name. It has always denoted a yellow stone, and
at the present day is applied by jewellers indis-
criminately to the true topaz and citrine, the yellow
quartz, the former, however, being sometimes dis-
tinguished by the prefix ' Brazilian.' ' Oriental '
topaz is corundum, and 'occidental' topaz is a
term occasionally employed for the yellow quartz.
Emerald, which means green, was first used for
chrysocolla, an opaque greenish stone (p. 288), but
was afterwards applied to the priceless green variety
of beryl, for which it is still retained. ' Oriental '
emerald is corundum, c Brazilian ' emerald in the
eighteenth century was a common term for the
green tourmaline recently introduced to Europe, and
' Uralian ' emerald has been tentatively suggested
for the green garnet more usually known as
'olivine.' Amethyst is properly the violet quartz,
but with the prefix ' oriental ' it is also applied to
violet corundum, though some jewellers, use it for
the brilliant quartz, with purple and white sectors,
from Siberia. Almandine, which is derived from the
name of an Eastern mart for precious stones, has
ii2 GEM-STONES
come to signify a stone of columbine-red hue,
principally garnet, but with suitable qualification
corundum and spinel also.
The nomenclature of jewellery tends to suggest
relations between the gem-stones for which there is
no real foundation, and to obscure the essential
identity, except from the point of view of colour,
of sapphire and ruby, emerald and aquamarine,
cairngorm and amethyst.
CHAPTER XIV
MANUFACTURED STONES r
THE initial step in the examination of a
crystallized substance is to determine its
physical characters and to resolve it by chemical
analysis into its component elements ; the final, and
by far the hardest, step is to build it up or synthetic-
ally prepare it from its constituents. Unknown to
the world at large, work of the latter kind has long
been going on within the walls of laboratories, and
as the advance in knowledge placed in the hands
of experimenters weapons more and more compar-
able with those wielded by nature, their efforts have
been increasingly successful. So stupendous, how-
ever, are the powers of nature that the possibility
of reproducing, by human agency, the treasured
stones which are extracted from the earth in various
parts of the globe at the cost of infinite toil and
labour has always been derided by those ignorant
of what had already been accomplished. Great,
therefore, was the consternation and the turmoil
when concrete evidence that could not be gainsaid
showed that man's restless efforts to bridle nature
to his will were not in vain, and congresses of
all the high-priests of jewellery were hastily con-
vened to ban such unrighteous products, with what
ultimate success remains to be seen.
8 "3
114 GEM-STONES
Crystallization may be caused in four different
ways, of which the second alone has as yet yielded
stones large enough to be cut —
1. By the separation of the substance from a
saturated solution. In nature the solvent may not
be merely hot water, or water charged with an acid,
but molten rock, and the temperature and the
pressure may be excessively high.
2. By the solidification of the liquefied substance
upon cooling. Ice is a familiar example of this
type.
3. By the sublimation of the vapour of the sub-
stance, which means the direct passage from the
vapour to the solid state without traversing the
usually intervening liquid state. It is usually the
most difficult of attainment of the four methods ;
the most familiar instance is snow.
4. By the precipitation of the substance from a
solution when set free by chemical action.
Other things being equal, the simpler the com-
position the greater is the ease with which a sub-
stance may be expected to be formed ; for, instead
of one complex substance, two or more different
substances may evolve, unless the conditions are
nicely arranged. Attempts, for instance, to produce
beryl might result instead in a mixture of chryso-
beryl, phenakite, and quartz.
By far the simplest in composition of all the
precious stones is diamond, which is pure crystallized
carbon ; but its manufacture is attended by well-
nigh insuperable difficulties. If carbon be heated
in air, it burns at a temperature well below its
melting point ; moreover, unless an enormously
high pressure is simultaneously applied, the product
MANUFACTURED STONES 1 1 5
is the other form of crystallized carbon, namely, the
comparatively worthless graphite. Moissan's in-
teresting course of experiments were in some degree
successful, but the tiny diamonds were worthless
as jewels, and the expense involved in their manu-
facture was out of all proportion to any possible
commercial value they might have.
Next to diamond the simplest substances among
precious stones are quartz (crystallized silica) and
corundum (crystallized alumina). The crystallization
of silica has been effected in several ways, but the
value in jewellery of quartz, even of the violet
variety, amethyst, is not such as to warrant its
manufacture on a commercial scale. Corundum,
on the other hand, is held in high esteem ; rubies
and sapphires, of good colour and free from flaws,
have always commanded good prices. The question
of their production by artificial means has therefore
more than academic interest.
Ever since the year 1837, when Gaudin produced
a few tiny flakes, French experimenters have steadily
prosecuted their researches in the crystallization of
corundum. Frdmy and Feil, in 1877, were the
first to meet with much success. A portion of one
of their crucibles lined with glistening ruby flakes
is exhibited in the British Museum (Natural
History).
In 1885 the jewellery market was completely
taken by surprise by the appearance of red stones,
emanating, so it is alleged, from Geneva ; having
the physical characters of genuine rubies, they were
accepted as, and commanded the prices of, the
natural stones. It was eventually discovered that
they had resulted from the fusion of a number of
n6
GEM-STONES
fragments of natural rubies in the oxy-hydrogen
flame. The original colour was driven off at that
high temperature, but was revived by the previous
addition of a little bichromate of potassium. Owing
to the inequalities of growth, the cracks due to
rapid cooling, the inclusion of
air-bubbles, often so numerous
as to cause a cloudy appear-
ance, and, above all, the un-
natural colour, these recon-
structed stones, as they are
termed, were far from satisfac-
tory, but yet they marked such
an advance on anything that
had been accomplished before
that for some time no suspicion
was aroused as to their being
other than natural stones.
A notable advance in the
synthesis of corundum, par-
ticularly of ruby, was made in
1 904, when Verneuil, who had
served his apprenticeship to
science under the guidance of
Fremy, invented his ingenious
inverted form of blowpipe
(Fig- 53)> which enabled him
to overcome the difficulties that had baffled earlier
investigators, and to manufacture rubies vying
in appearance after cutting with the best of
nature's productions. The blowpipe consisted of
two tubes, of which the upper, E, wide above, was
constricted below, and passing down the centre
of the lower, F, terminated just above the orifice
FIG. 53. — Verneuil's In-
verted Blowpipe.
MANUFACTURED STONES 117
of the latter in a fine nozzle. Oxygen was admitted
at C through the plate covering the upper end of
the tube, E. A rod, which passed through a rubber
collar in the same plate, supported inside the tube,
E, a vessel, D, and at the upper end terminated in
a small plate, on which was fixed a disc, B. The
hammer, A, when lifted by the action of an electro-
magnet and released, fell by gravity and struck the
disc. The latter could be turned about a horizontal
axis placed eccentrically, so that the height through
which the hammer fell and the consequent force of
the blow could be regulated. The rubber collar,
which was perfectly gas-tight, held the rod securely,
but allowed the shocks to be transmitted to the
vessel, D, an arrangement of guides maintaining
the slight motion of the vessel strictly vertical.
This vessel, which carried the alumina powder used
in the manufacture of the stone, had as its base a
cylindrical sieve of fine mesh. The succession of
rapid taps of the hammer caused a regular feed of
powder down the tube, the amount being regulated
by varying the height through which the hammer
fell. Hydrogen or coal-gas was admitted at G
into the outer tube, F, and in the usual way met
the oxygen just above the orifice, L. To exclude
irregular draughts, the flame was surrounded by a
screen, M, which was provided with a mica window,
and a water-jacket, K, protected the upper part of
the apparatus from excessive heating.
The alumina was precipitated from a solution of
pure ammonia - alum, (NH4)2SO4.A12(SO4)3.24H2O,
in distilled water by the addition of pure ammonia,
sufficient chrome-alum also being dissolved with
the ammonia-L^um to furnish about 2\ per cent.
1 1 8 GEM-STONES
of chromic oxide in the resulting stone. The
powder, carefully prepared and purified, was placed,
as has been stated above, in the vessel, D, and on
reaching the flame at the orifice it melted, and fell
as a liquid drop, N, upon the pedestal, P, which
was formed of previously fused alumina. This
pedestal was attached by a platinum sleeve to an
iron rod, Q, which was provided with the necessary
screw adjustments, R and S, for centring and
lowering it as the drop grew in size. Great care
was exercised to free the powder from any trace of
potassium, which, if present, imparted a brownish
tinge to the stone. The pressure of the oxygen,
low initially both to prevent the
pedestal from melting, and to keep
the area of the drop in contact with
the pedestal as small as possible,
FIG. 54.— 'Boule,' because otherwise flaws tended to
D Car S aped start on cooling, was gradually in-»
creased until the flame reached the
critical temperature which kept the top of the drop
melted, but not boiling. The supply of powder was at
the same time carefully proportioned to the pressure.
The pedestal, P, was from time to time lowered, and
the drop grew in the shape of a pear (Fig. 54), the apex
of which was downwards and adhered to the pedestal
by a narrow stalk. As soon as the drop reached
the maximum size possible with the size of the
flame, the gases were sharply and simultaneously
cut off. After ten minutes or so the drop was
lowered from the chamber, M, by the screw, S, and
when quite cold was removed from the pedestal.
Very few changes have been made in the method
when adapted to commercial use. Coal-gas has,
BI.OWI'II'E USED FOR THE MANUFACTURE OF RUBIES AND SAPPHIRES
MANUFACTURED STONES 119
however, entirely replaced the costly hydrogen, and
the hammer is operated by a cam instead of an
electromagnet, while, as may be seen from the view
of a gem-stone factory (Plate XIV), a number of blow-
pipes are placed in line so that their cams are
worked by the same shaft, a. The fire-clay screen,
b, surrounding the flame is for convenience of re-
moval divided into halves longitudinally, and a
small hole is left in front for viewing the stone
during growth, a red glass screen, c, being provided
in front to protect the eyes from the intense glare.
Half the fire-clay screen of the blowpipe in the
centre of the Plate has been removed to show the
arrangement of the interior. The centring and
the raising and lowering apparatus, d, have been
modified. The process is so simple that one man
can attend to a dozen or so of these machines, and
it takes only one hour to grow a drop large enough
to be cut into a ten-carat stone.
The drops, unless the finished stone is required
to have a similar pear shape, are divided longitudin-
ally through the central core into halves, which in
both shape and orientation are admirably suited to
the purposes of cutting ; as a general rule, the drop
splits during cooling into the desired direction of
its own accord.
Each drop is a single crystalline individual, and
not, as might have been anticipated, an alumina
glass or an irregular aggregation of crystalline
fragments, and, if the drop has cooled properly,
the crystallographic axis is parallel to the core of
the pear. The cut stone will therefore have not
only the density and hardness, but also all the
optical characters — refractivity, double refraction,
120 GEM-STONES
dichroism, etc. — pertaining to the natural species,
and will obey precisely the same tests with the re-
fractometer and the dichroscope. Were it not for
certain imperfections it would be impossible to
distinguish between the stones formed in Nature's
vast workshop and those produced
within the confines of a laboratory.
The artificial stones, however, are
rarely, if ever, free from minute
air-bubbles (Fig. 55), which can
FIG. 55.— Bubbles easily be seen with an ordinary
and Curved Strise lens Their spherical shape differ-
in Manufactured . , .. , ,
Ruby. entiates them from the plane-
sided cavities not infrequently
visible in a natural stone (Fig. 56). Moreover,
the colouring matter varies slightly, but imper-
ceptibly, in successive shells, and consequently in
the finished stone a careful eye can discern the
curved striations (Fig. 55) corresponding in shape
to the original shell. In a natural
stone, on the other hand, although
zones of different colours or varying
shades are not uncommon, the
resulting striations are straight
(Fig. 56), corresponding to the ^
plane faces of the original crystal in Naturai Ruby.
form. By sacrificing material it
might be possible to cut a small stone free from
bubbles, but the curved striations would always be
present to betray its origin.
The success that attended the manufacture of
ruby encouraged efforts to impart other tints to
crystallized alumina. By reducing the percentage
amount of chromic oxide, pink stones were turned
MANUFACTURED STONES 121
out, in colour not unlike those Brazilian topazes, the
original hue of which has been altered by the appli-
cation of heat. These artificial stones have there-
fore been called ' scientific topaz ' ; of course, quite
wrongly, since topaz, which is properly a fluo-silicate
of aluminium, is quite a different substance.
Early attempts made to obtain the exquisite blue
tint of the true sapphire were frustrated by an un-
expected difficulty. The colouring matter, cobalt
oxide, was not diffused evenly through the drop,
but was huddled together in splotches, and it was
found necessary to add a considerable amount of
magnesia as a flux before a uniform distribution of
colour could be secured. It was then discovered
that, despite the colour, the stones had the physical
characters, not of sapphire, but of the species closely
allied to it, namely, spinel, aluminate of magnesium.
By an unsurpassable effort of nomenclature these
blue stones were given the extraordinary name of
' Hope sapphire,' from fanciful analogy with the
famous blue diamond which was once the pride of
the Hope collection. A blue spinel is occasionally
found in nature, but the actual tint is somewhat
different. These manufactured stones have the
disadvantage of turning purple in artificial light.
By substituting lime for magnesia as a flux, Paris,
a pupil of Verneuil's, produced blue stones which
were not affected to the same extent. The difficulty
was at length overcome at the close of 1909, when
Verneuil, by employing as tinctorial agents 0*5 per
cent, of titanium oxide and 1-5 per cent, of magnetic
iron oxide, succeeded in producing blue corundum ;
it, however, had not quite the tint of sapphire.
Stones subsequently manufactured, which were
122 GEM-STONES
better in colour, contained about 0-12 per cent
of titanium oxide, but no iron at all.
By the addition to the alumina of a little nickel
oxide and vanadium oxide respectively, yellow and
yellowish green corundums have been obtained.
The latter have in artificial light a distinctly reddish
hue, and have therefore been termed 'scientific
alexandrite'; of course, quite incorrectly, since the
true alexandrite is a variety of chrysoberyl, alumin-
ate of beryllium, a very different substance.
If no colouring matter at all be added and the
alum be free from potash, colourless stones or white
sapphires are formed, which pass under the name
' scientific brilliant.' It is scarcely necessary to
remark that they are quite distinct from the true
brilliant, diamond.
The high prices commanded by emeralds, and
the comparative success that attended the recon-
struction of ruby from fragments of natural stones,
suggested that equal success might follow from a
similar process with powdered beryl, chromic oxide
being used as the colouring agent. The resulting
stones are, indeed, a fair imitation, being even pro-
vided with flaws, but they are a beryl glass with
lower specific gravity and refractivity than the true
beryl, and are wrongly termed ' scientific emerald.'
Moreover, recently most of the stones so named on
the market are merely green paste.
It is unfortunate that the real success which has
been achieved in the manufacture of ruby and sap-
phire should be obscured by the ill-founded claims
tacitly asserted in other cases.
At the time the manufactured ruby was a novelty
it fetched as much as £,6 a carat, but as soon as
MANUFACTURED STONES 123
it was discovered that it could easily be differenti-
ated from the natural stone, a collapse took place,
and the price fell abruptly to 305., and eventually
to 5s. and even is. a carat. The sapphires run
slightly higher, from 2s. to ?s. a carat. The prices
of the natural stones, which at first had fallen, have
now risen to almost their former level. The extreme
disparity at present obtaining between the prices
of the artificial and the natural ruby renders the
fraudulent substitution of the one for the other
a great temptation, and it behoves purchasers to
beware where and from whom they buy, and to be
suspicious of apparently remarkable bargains, especi-
ally at places like Colombo and Singapore where
tourists abound. It is no secret that some thousands
of carats of manufactured rubies are shipped annu-
ally to the East. Caveat emptor.
CHAPTER XV
IMITATION STONES
THE beryl glass mentioned in the previous
chapter marks the transition stage between
manufactured stones which in all essential characters
are identical with those found in nature, and arti-
ficial stones which resemble the corresponding natural
stone in outward appearance only. In a sense both
sorts may be styled artificial, but it would be mis-
leading to confound them under the same appel-
lation.
Common paste,1 which is met with in drapery
goods and cheap ornaments in general — hat-pins,
buckles, and so forth — is composed of ordinary
crown-glass or flint-glass, the refractive indices
being about 1*53 and i'63 respectively. The
finest quality, which is used for imitations of
brilliants, is called ' strass.' It is a dense lead flint-
glass of high refraction and strong colour - dis-
persion, consisting of 38*2 per cent, of silica, 53^3
red lead (oxide of lead), and 7'8 potassium carbon-
ate, with small quantities of soda, alumina, and
other substances. How admirable these imitations
may be, a study of the windows of a shop devoted
1 The word paste is derived from the Italian, pasta, food, being
suggested by the soft plastic nature of the material used to imitate
gems.
IMITATION STONES 125
to such things will show. Unfortunately the
addition of lead, which is necessary for imparting
the requisite refraction and ' fire ' to the strass,
renders the stones exceedingly soft. All glass
yields to the file, but strass stones are scratched
even by ordinary window-glass. If worn in such
a way that they are rubbed, they speedily lose the
brilliance of their polish, and, moreover, they are
susceptible to attack by the sulphurous fumes
present in the smoky air of towns, and turn after
a time a dirty brown in hue. When coloured
stones are to be imitated, small quantities of a
suitable metallic oxide are fused with the glass ;
cobalt gives rise to a royal-blue tint, chromium a
ruby red, and manganese a violet. Common paste
is not highly refractive enough to give satisfactory
results when cut as a brilliant, and the bases are
therefore often coated with quicksilver, or, in the case
of old jewellery, covered with foil in the setting, in
order to secure more complete reflection from the
interior. The fashioning of these imitation stones
is easy and cheap. Being moulded, they do not
require cutting, and the polishing of the facets thus
formed is soon done on account of the softness of
the stones.
A test with a file readily differentiates paste
stones from the natural stones they pretend to be.
Being necessarily singly refractive, they are, of
course, lacking in dichroism, and their refractivity
seldom accords even approximately with that of
the corresponding natural stone.
In order to meet the test for hardness the
doublet was devised. Such a stone is composed
of two parts — the crown consisting of colourless
126 GEM-STONES
quartz or other inexpensive real and hard stone,
and the base being made up of coloured glass.
When the imitation, say of a sapphire, is intended
to be more exact, the crown is made of a real
sapphire, but one deficient in colour, the requisite
tint being obtained from the paste forming the
under part of the doublet. In case the base
should also be tested for hardness the triplet
has been devised. In this the base is made of a
real stone also, and the coloured paste is confined
to the girdle section, where it is hidden by the
setting. Sapphires and emeralds of indifferent
colour are sometimes slit across the girdle; the
interior surfaces are polished, and colouring matter
is introduced with the cement, generally Canada
balsam, which is used to re-unite the two portions
of the stone together. All such imitations may
be detected by placing the stone in oil, when the
surfaces separating the portions of the composite
stone will be visible, or the binding cement may
be dissolved by immersing the stone, if unmounted,
in boiling water, or in alcohol or chloroform, when
the stone will fall to pieces.
The glass imitations of pearls, which have be-
come very common in recent years, may, apart from
their inferior iridescence, be detected by their greater
hardness, or by the apparent doubling of, say, a spot
of ink placed on the surface, owing to reflection from
the inner surface of the glass shell. They are made
of small hollow spheres formed by blowing. Next
to the glass comes a lining of parchment size, and
next the under lining, which is the most important
part of the imitation, consisting of a preparation of
fish scales called Essence d' Orient. When the lining
IMITATION STONES 127
is dry, the globe is filled with hot wax to impart the
necessary solidity. In cheap imitations the glass
balls are not lined at all, but merely heated with
hydrochloric acid to give an iridescence to the sur-
face ; sometimes they are coated with wax, which can
be scraped off with a knife.
PART II— SECTION A
PRECIOUS STONES
CHAPTER XVI
DIAMOND
DIAMOND has held pride of place as chief
of precious stones ever since the discovery
of the form of cutting known as the ' brilliant '
revealed to full perfection its amazing qualities ; and
justly so, since it combines in itself extreme hardness,
high refraction, large colour-dispersion, and brilliant
lustre. A rough diamond, especially from river
gravels, has often a peculiar greasy appearance, and
is no more attractive to the eye than a piece of
washing-soda. It is therefore easy to understand
why the Persians in the thirteenth century placed
the pearl, ruby, emerald, and even peridot before it,
and writers in the Middle Ages frequently esteemed
it below emerald and ruby. The Indian lapidaries,
who were the first to realize that diamond could
be ground with its own powder, discovered what
a wonderful difference the removal of the skin makes
in the appearance of a stone. They, however,
made no attempt to shape a stone, but merely
polished the natural facets, and only added numerous
DIAMOND 129
small facets when they wished to conceal flaws or
other imperfections ; indeed, the famous traveller,
Tavernier, from whom most of our knowledge of
early mining in India is obtained, invariably found
that a stone covered with many facets was badly
flawed. The full radiant beauty of a diamond
comes to light only when it is cut in brilliant form.
Of all precious stones diamond has the simplest
composition ; it is merely crystallized carbon, another
form of which is the humble and useful graphite,
commonly known as ' black-lead.' Surely nature
has surpassed all her marvellous efforts in producing
from the same element substances with such
divergent characters as the hard, brilliant, and
transparent diamond and the soft, dull, and opaque
graphite. It is, however, impossible to draw any
sharp dividing line between the two ; soft diamond
passes insensibly into hard graphite, and vice versa.
Boart, or bort, as it is sometimes written, is composed
of minute crystals of diamond arranged haphazardly ;
it possesses no cleavage, its hardness is greater than
that of the crystals, and its colour is greyish to
blackish. Carbon, carbonado, or black diamond,
which is composed of still more minute crystals,
is black and opaque, and is perceptibly harder
than the crystals. It passes into graphite, which
varies in hardness, and may have any density
between 2-o and 3-o. Jewellers apply the term
boart to crystals or fragments which are of no
service as gems ; such pieces are crushed to powder
and used for cutting and polishing purposes.
Diamonds, when absolutely limpid and free from
flaws, are said to be of the ' first water,' and are
most prized when devoid of any tinge of colour
9
1 30 GEM-STONES
except perhaps bluish (Plate I, Fig. i). Stones with
a slight tinge of yellow are termed 'off-coloured,'
and are far less valuable. Those of a canary-yellow
colour (Plate I, Fig. 3), however, belong to a different
category, and have a decided attractiveness. Green-
ish stones also are common, though it is rare to
come across one with a really good shade of that
colour. Brown stones, especially in South Africa,
are not uncommon. Pink stones are less common,
and ruby-red and blue stones are rare. Those of
the last-named colour have usually what is known
FIGS. 57-59.— Diamond Crystals.
as a ' steely ' shade, i.e. they are tinged with green ;
stones of a sapphire blue are very seldom met with,
and such command high prices.
Diamond crystallizes (Figs. 57—59 and Plate I,
Fig. 2) in octahedra with brilliant, smooth faces, and
occasionally in cubes with rough pitted faces ;
sometimes three or six faces take the place of each
octahedron face, and the stone is almost spherical
in shape. The surfaces of the crystals are often
marked with equilateral triangles, which are supposed
to represent the effects of incipient combustion.
Twinned crystals, in which the two individuals
may be connected by a single plane or may be
DIAMOND 131
interpenetrating, a star shape often resulting in the
latter case, are common ; sometimes, if of the
octahedron type, they are beautifully symmetrical.
The rounded crystals are frequently covered with
a peculiar gum-like skin which is somewhat less
hard than the crystal itself. A large South African
stone, weighing 27 grams (130 carats) and octahedral
in shape, which was the gift of John Ruskin, and
named by him the ' Colenso ' after the first bishop of
Natal, is exhibited in the British Museum (Natural
History) ; its appearance is, however, marred by its
distinctly ' off-coloured ' tint.
The refraction of diamond is single, but local
double refraction is common, indicating a state of
strain which can often be traced to an included
drop of liquid carbonic acid ; so great is the strain
that many a fine stone has burst to fragments on
being removed from the ground in which it has lain.
The refractive index for the yellow light of a sodium
flame is 2^4 17 5, and the slight variation from this
mean value that has been observed, amounting only
to O'OOOl, testifies to the purity of the composition.
The colour-dispersion is large, being as much as
0-044, in which respect it surpasses all colourless
stones, but is exceeded by sphene and the green
garnet from the Urals (cf. p. 217). The lustre of
diamond, when polished, is so characteristic as to
be termed adamantine, and is due to the combina-
tion of high refraction and extreme hardness.
Diamond is translucent to the X (Rontgen) rays ;
it phosphoresces under the action of radium, and
of a high-tension electric current when placed in
a vacuum tube, and sometimes even when exposed
to strong sunlight. Some diamonds fluoresce in
132 GEM-STONES
sunlight, turning milky, and a few even emit light
when rubbed. Crookes found that a diamond
buried in radium bromide for a year had acquired
a lovely blue tint, which was not affected even by
heating to redness. The specific gravity is like-
wise constant, being 3*521, with a possible variation
from that mean value of 0*005 5 but a greater range,
as might be expected, is found in the impure boart.
Diamond is by far the hardest substance in nature,
being marked I o in Mohs's scale of hardness, but
it varies in itself; stones from Borneo and New
South Wales are so perceptibly harder than those
usually in the lapidaries' hands, that they can be
cut only with their own and not ordinary diamond
powder, and some difficulty was experienced in
cutting them when they first came into the market.
It is interesting to note that the metal tantalum,
the isolation of which in commercial amount
constituted one of the triumphs of chemistry of
recent years, has about the same hardness as
diamond. Despite its extreme hardness diamond
readily cleaves under a heavy blow in planes
parallel to the faces of the regular octahedron, a
property utilized for shaping the stone previous to
cutting it. The fallacious, but not unnatural, idea
was prevalent up to quite modern times that a
diamond would, even if placed on an anvil, resist
a blow from a hammer : who knows how many
fine stones have succumbed to this illusory test?
The fact that diamond could be split was known to
Indian lapidaries at the time of Tavernier's visit,
and it would appear from De Boodt that in the
sixteenth century the cleavability of diamond was
not unknown in Europe, but it was not credited
DIAMOND 133
at the time and was soon forgotten. Early last
century Wollaston, a famous chemist and mineral-
ogist, rediscovered the property, and, so it is said,
used his knowledge to some profit by purchasing
large stones, which because of their awkward shape
or the presence of flaws in the interior were rejected
by the lapidaries, and selling them back again after
cleaving them to suitable forms.
It has already been remarked (p. 79) that the
interval in hardness between diamond and corundum,
which comes next to it in Mohs's scale, is enormously
greater than that between corundum and the softest
of minerals. Diamond can therefore be cut only
with the aid of its own powder, and the cutting of
diamond is therefore differentiated from that of other
stones, the precious-stone trade being to a large ex-
tent divided into two distinct groups, namely, dealers
in diamonds, and dealers in all other gem-stones.
The name of the species is derived from the
popular form, adiamentem, of the Latin adamantem,
itself the alliterative form of the Greek aSa/^a?,
meaning the unconquerable, in allusion not merely
to the great hardness but also to the mistaken idea
already mentioned. Boart probably comes from
the Old-French bord or borty bastard.
At the present day diamonds are usually cut as
brilliants, though the contour of the girdle may be
circular, oval, or drop-shaped to suit the particular
purpose for which the stone is required, or to keep
the weight as great as possible. Small stones for
bordering a large coloured stone may also be cut
as roses or points. A perfect brilliant has 5 8 facets,
but small stones may have not more than 44, and
exceptionally large stones may with advantage have
134 GEM-STONES
many more ; for instance, on the largest stone cut
from the Cullinan diamond there are no fewer than
74 facets.
The description of the properties of diamond
would not be complete without a reference to the
other valuable, if utilitarian, purposes to which it is
put Without its aid much of modern engineering
work and mining operations would be impossible
except at the cost of almost prohibitive expenditure
of time and money.
Boring through solid rock has been greatly
facilitated by the use of the diamond drill. For
this purpose carbonado or black diamond is more
serviceable than single crystals, and the price of
the former has consequently advanced from a
nominal figure up to £3 to £12 a carat. The actual
working part of the drill consists of a cast-steel
ring. The crown of it has a number of small
depressions at regular intervals into which the
carbonados are embedded. On revolution of the
drill an annular ring is cut, leaving a solid core
which can be drawn to the surface. For cooling
the drill and for washing away the detritus water
is pumped through to the working face. The
duration of the carbonados depends on the nature
of the rock and the skill of the operator. The most
troublesome rock is a sandstone or one with sharp
differences in hardness, because the carbonados are
liable to be torn out of their setting. An experienced
operator can tell by the feel of the drill the nature
of the rock at the working face, and by varying the
pressure can mitigate the risk of damage to the drill.
The tenacity of diamond renders it most suitable
for wire-drawing. The tungsten filaments used in
DIAMOND 135
many of the latest forms of incandescent electric
lamps are prepared in this manner.
Diamond powder is used for cutting and turning
the hardened steel employed in modern armaments
and for other more peaceful purposes.
Although nearly all the gem-stones scratch glass,
diamond alone can be satisfactorily employed to
cut it along a definite edge. Any flake at random
will not be suitable, because it will tear the glass
and form a jagged edge. The best results are given
by the junction of two edges which do not meet in
too obtuse an angle ; two edges of the rhombic
dodecahedron meet the requirements admirably.
The stones used by the glaziers are minute in size,
being not much larger than .a pin's head, and thirty
of them on an average go to the carat. They are
set in copper or brass. Some little skill is needed
to obtain the best results.
The value of a diamond has always been determined
largely by the size of the stone, the old rule being
that the rate per carat should be multiplied by the
square of the weight in carats ; thus, if the rate be
£ i o, the cost of a two-carat stone is four times this
sum, or £4.0, of a three-carat stone £90, and so on.
For a century, from 1750 to 1850, the rate remained
almost constant at £4. for rough, £6 for rose-cut,
and £8 for brilliant-cut diamonds. Since the latter
date, owing to the increase in the supply of gold,
the growth of the spending power of the world, and
the gradual falling off in the productiveness of
the Brazilian fields, the rate steadily increased about
10 per cent, each year, until in 1865 the rate for
brilliants was £iB. The rise was checked by
the discovery of the South African mines ; moreover,
136 GEM-STONES
since comparatively large stones are plentiful in
these mines, the rule of the increase in the price of
a stone by the square of its weight no longer holds.
The rate for the most perfect stones still remains
high, because such are not so common in the South
African mines. The classification 1 adopted by the
syndicate of London diamond merchants who place
upon the market the output of the De Beers group
of mines is as follows : — (a) Blue-white, (b} white,
(c) silvery Cape, (d) fine Cape, (e) Cape, (/") fine by-
water, (g-) by water, (/t) fine light brown, (z) light
brown, (/) brown, (/£) dark brown. Bywaters or
byes are stones tinged with yellow.
The rate per carat for cut stones in the blue-
white and the by water groups is : —
BLUE-WHITE. BYWATER.
5-carat stone . . ^40-60 ^20-25
i „ . 30-40 10-15
\ „ . . 20-25 8-12
J „ . 15-18 6-10
Melee . . . 12-15 5-8
Melee are stones smaller than a quarter of a carat.
It will be noticed that the prices depart largely
from the old rule ; thus taking the rate for a carat
blue-white stone, the price of a five-carat stone
should be from £15 0—200 a carat, and for a
quarter-carat stone only £7, los. to £10 a carat.
There happens to be at the time of writing very
little demand for five-carat stones. Of course, the
prices given are subject to constant fluctuation
depending upon the supply and demand, and the
whims of fashion.
1 Cf. below, p. 149.
CHAPTER XVII
OCCURRENCE OF DIAMOND
THE whole of the diamonds known in ancient
times were obtained from the so-called
Golconda mines in India. Golconda itself, now a
deserted fortress near Hyderabad, was merely the
mart where the diamonds were bought and sold.
The diamond-bearing district actually spread over a
wide area on the eastern side of the Deccan, ex-
tending from the Pinner River in the Madras
Presidency northwards to the Rivers Son and Khan,
tributaries of the Ganges, in Bundelkhand. The
richest mines, where the large historical stones were
found, are in the south, mostly near the Kistna
River. The diamonds were discovered in sandstone,
or conglomerate, or the sands and gravels of river-
beds. The mines were visited in the middle of the
seventeenth century by the French traveller and
jeweller, Tavernier, when travelling on a commission
for Louis XIV, and he afterwards published a careful
description of them and of the method of working
them. The mines seem to have been exhausted in
the seventeenth century ; at any rate, the prospecting,
which has been spasmodically carried on during the
last two centuries, has proved almost abortive.
With the exception of the Koh-i-nor, all the large
Indian diamonds were probably discovered not long
138 GEM-STONES
before Tavernier's visit. The diamonds known to
Pliny, and in his time, were quite small, and it is
doubtful if any stones of considerable size came to
light before A.D. 1000.
India enjoyed the monopoly of supplying the
world's demand for diamonds up to the discovery,
in 1725, of the precious stone in Brazil. Small
stones were detected by the miners in the gold
washings at Tejuco, about eighty miles (129 km.)
from Rio de Janeiro, in the Serro do Frio district of
the State of Minas Geraes. The discovery naturally
caused great excitement. So many diamonds were
found that in 1727 something like a slump took
place in their value. In order to keep up prices,
the Dutch merchants, who mainly controlled the
Indian output, asserted that the diamonds had not
been found in Brazil at all, but were inferior Indian
stones shipped to Brazil from Goa. The tables
were neatly turned when diamonds were actually
shipped from Brazil to Goa, and exported thence to
Europe as Indian stones. This course and the
continuous development of the diamond district in
Brazil rendered it impossible to hoodwink the world
indefinitely. The drop in prices was, however,
stayed by the action of the Portuguese government,
who exacted such heavy duties and imposed such
onerous conditions that finally no one would under-
take to work the mines. Accordingly, in 1772
diamond-mining was declared a royal monopoly in
Brazil, and such it remained until the severance of
Brazil from Portugal in 1834, when private mining
was permitted by the new government subject to the
payment of reasonable royalties. The industry was
enormously stimulated by the discovery, in 1 844, of
OCCURRENCE OF DIAMOND 139
the remarkably rich fields in the State of Bahia,
especially at Serra da Cincora, where carbonado,
or black diamond, first came to light, but after a-
few years, owing to the difficulties of supplying
labour, the unhealthiness of the climate, and the
high cost of living, the yield fell off and gradually
declined, until the importance of the fields was finally
eclipsed by the rise of the South African mines.
The Brazilian mines have proved very productive,
but chiefly in small diamonds, stones above a carat
in weight being few in comparison. The largest
stone, to which the name, the Star of the South,
was applied, weighed in the rough 254^ carats; it
was discovered at the Bagagem mines in 1853.
The quality of the diamonds is good, many of them
having the highly-prized bluish-white colour. The
principal diamond-bearing districts of Brazil centre
at Diamantina, as Tejuco was re-named after the
discovery of diamonds, Grao Magor, and Bagagem
in the State of Minas Geraes, at Diamantina in the
State of Bahia, and at Goyaz and Matto Grosso in
the States of the same names. The diamonds
occur chiefly in cascalho, a gravel, containing large
masses of quartz and small particles of gold, which
is supposed to be derived from a quartzose variety
of micaceous slate known as itacolumite. The
mines are now to some extent being worked by
systematic dredging of the river-beds.
Early in 1867 the children of a Boer farmer,
Daniel Jacobs, who dwelt near Hopetown on the
banks of the Orange River, picked up in the course
of play near the river a white pebble, which was
destined not only to mark the commencement of a
new epoch in the record of diamond mines, but to
140 GEM-STONES
change the whole course of the history of South
Africa. This pebble attracted the attention of a
neighbour, Schalk van Niekerk, who suspected that
it might be of some value, and offered to buy it.
Mrs. Jacobs, however, gave it him, laughingly scout-
ing the idea of accepting money for a mere pebble.
Van Niekerk showed it to a travelling trader, by
name John O'Reilly, who undertook to obtain what
he could for it on condition that they shared the
proceeds. Every one he met laughed to scorn the
idea that the stone had any value, and it was once
thrown away and only recovered after some search
in a yard, but at length he showed it to Lorenzo
Boyes, the Acting Civil Commissioner at Colesberg,
who, from its extreme hardness, thought it might be
diamond and sent it to the mineralogist, W. Guybon
Atherston, of Grahamstown, for determination. So
uncertain was Boyes of its value that he did not
even seal up the envelope containing it, much less
register the package. Atherston found immediately
that the long-scorned pebble was really a fine
diamond, weighing 21^- carats, and with O'Reilly's
consent he submitted it to Sir Philip Wodehouse,
Governor at the Cape. The latter purchased it at
once for £500, and dispatched it to be shown at
the Paris Exhibition of that year. It did not,
however, attract much attention ; chimerical tales
of diamond finds in remote parts of the world are
not unknown. Indeed, for some time only a few
small stones were picked up beside the Orange
River, and no one believed in the existence of any
extensive diamond deposit. However, all doubt as
to the advisibility of prospecting the district was
settled by the discovery of the superb diamond,
PLATE
-.
PLATE XVI
OCCURRENCE OF DIAMOND 141
afterwards known as the ' Star "of South Africa,'
which was picked up in March 1869 by a shepherd
boy on the Zendfontein farm near the Orange River.
Van Niekerk, on the alert for news of further dis-
coveries, at once hurried to the spot and purchased
the stone from the boy for five hundred sheep, ten
oxen, and a horse, which seemed to the boy untold
wealth, but was not a tithe of the £11,200 which
Lilienfeld Bros., of Hopetown, gave Van Niekerk.
This remarkable discovery attracted immediate
attention to the potentialities of a country which
produced diamonds of such a size, and prospectors
began to swarm into the district, gradually spread-
ing up the Vaal River. For some little time not
much success was experienced, but at length, early
in 1870, a rich find was made at Klipdrift, now
known as Barkly West, which was on the banks of
the Vaal River immediately opposite the Mission
camp at Pniel. The number of miners steadily
increased until the population on the two sides of
the river included altogether some four or five
thousand people, and there was every appearance
of stability in the existing order of things. But a
vast change came over the scene upon the discovery
of still richer mines lying to the south-east and some
distance from the river. The ground was actually
situated on the route traversed by parties hurrying
to the Vaal River, but no one dreamed of the wealth
that lay under their feet. The first discovery was
made in August 1870 at the farm Jagersfontein,
near Fauresmith in Orange River Colony, by De
Klerk, the intelligent overseer, who noticed in the dry
bed of a stream a number of garnets, and, knowing
that they often accompanied diamond, had the curi-
142 GEM-STONES
osity to investigate the point. He was immediately
rewarded by finding a fine diamond weighing 50
carats. In the following month diamonds were
discovered about twenty miles from Klipdrift at
Dutoitspan on the Dorstfontein farm, and a little
later also on the contiguous farm of Bultfontein ; a
diamond was actually found in the mortar used in
the homestead of the latter farm. Early in May
1871 diamonds were found about two miles away
on De Beers' farm, Vooruitzigt, and two months
later, in July, a far richer find was made on the
same farm at a spot which was first named Coles-
berg Kopje, the initial band of prospectors having
come from the town of that name near the Orange
River, but was subsequently known as Kimberley
after the Secretary of State for the Colonies at that
time. Soon a large and prosperous town sprang
up close to the mines ; it rapidly grew in size and
importance, and to this day remains the centre of
the diamond-mining industry. Subsequent pro-
specting proved almost blank until the discovery
of the Premier or Wesselton mine on Wesselton
farm, about four miles from Kimberley, in September
1890; it received the former name after Rhodes,
who was Premier of Cape Colony at that date. No
further discovery of any importance was made until,
in 1902, diamonds were found about twenty miles
north-west-north of Pretoria in the Transvaal, at the
new Premier mine, now famed as the producer of
the gigantic Cullinan diamond.
The Kimberley mines were at first known as the
' dry diggings ' on account of their arid surround-
ings in contradistinction to the 'river diggings ' by
the Vaal. The dearth of water was at first one of
PLATE Xl'lII
_o&jJi£a*-*L ** ?>d ,^/ i "*_'&'* *.! „
KIMBERLEY MINE, l83l
OCCURRENCE OF DIAMOND 143
the great difficulties in the way of working the
former mines, although subsequently the accumula-
tion of underground water at lower levels proved a
great obstacle to the working of the mines. The
' river diggings ' were of a type similar to that met
with in India and Brazil, the diamonds occurring
in a gravelly deposit of limited thickness beneath
which was barren rock, but the Kimberley mines
presented a phenomenon hitherto without precedent
in the whole history of diamond mining. The
diamonds were found in a loose surface deposit,
which was easily worked, and for some time the
prospectors thought that the underlying limestone
corresponded to the bedrock of the river gravel,
until at length one more curious than his fellows
investigated the yellowish ground underneath, and
found to his surprise that it was even richer than
the surface layer. Immediately a rush was made
back to the deserted claims, and the mines were
busier than ever. This ' yellow ground,' as it is
popularly called, was much decomposed and easy,
therefore, to work and sift. About fifty to sixty
feet (15—18 m.) below the surface, however, it
passed into a far harder rock, which from its colour
is known as the ' blue ground ' ; this also, to the
unexpected pleasure of the miners, turned out to
contain diamonds. Difficulties arose as each claim,
30 by 30 Dutch feet (about 31 English feet or
9-45 metres square) in area, was worked downwards.
In the Kimberley mine (Plate XVI) access to the vari-
ous claims was secured by retaining parallel strips,
i 5 feet wide, each claim being, therefore, reduced in
width to 22| feet, to form roadways running from
side to side of the mine in one direction. These,
144 GEM-STONES
however, soon gave way, not only because of the
falling of the earth composing them, but because
they were undermined and undercut by the owners
of the adjacent claims. By the end of 1872 the
last roadway had disappeared, and the mine pre-
sented the appearance of a vast pit. In order to
obtain access to the claims without intruding on
those lying between, and to provide for the hauling
of the loads of earth to the surface, an ingenious
system of wire cables in three tiers (Plate XVII) was
erected, the lowest tier being connected to the outer-
most claims, the second to claims farther from the
edge, and the highest to claims in the centre of
the pit. The mine at that date presented a most
remarkable spectacle, resembling an enormous
radiating cobweb, which had a weird charm by
night as the moonlight softly illuminated it, and by
day, owing to the perpetual ring of the flanged
wheels of the trucks on the running wires, twanged
like some gigantic aeolian harp. This system ful-
filled its purpose admirably until, with increasing
depth of the workings, other serious difficulties arose.
Deprived of the support of the hard blue ground,
the walls of the mine tended to collapse, and
additional trouble was caused by the underground
water that percolated into the mine. By the end of
1883 the floor of the Kimberley mine was almost
entirely covered by falls of reef (Plate XVIII), as the
surrounding rocks are termed, the depth then being
about 400 feet (122 m.). In the De Beers mine, in
spite of the precaution taken to prevent falls of reef
by cutting the walls of the mine back in terraces, falls
occurred continuously in 1884, and by 1887, at a
depth of 350 feet (107 m.), all attempts at open work-
PLATE XIX
PLATE XX
OCCURRENCE OF DIAMOND 145
ing had to be abandoned. In the Dutoitspan mine
buttresses of blue ground were left, which held back
the reef for some years, but ultimately the mine
became unsafe, and in March 1886 a disastrous
fall took place, in which eighteen miners — eight
white men and ten Kafirs — lost their lives. The
Bultfontein mine was worked to the great depth of
500 feet (152 m.), but falls occurred in 1889 and put
an end to open working. In all cases, therefore, the
ultimate end was the same : the floor of the mine
became covered with a mass of worthless reef, which
rendered mining from above ground dangerous,
and, indeed, impossible except at prohibitive cost.
It was then clearly necessary to effect access to
the diamond-bearing ground by means of shafts
sunk at a sufficient distance from the mine to re-
move any fear of falls of reef. For such schemes
co-operative working was absolutely essential. Plate
XIX illustrates the desolate character of the Kimber-
ley mine above ground and the vastness of the
yawning pit, which is over 1000 feet (300 m.) in
depth.
A certain amount of linking up of claims had
already taken place, but, although many men must
have seen that the complete amalgamation of the
interests in each mine was imperative, two men
alone had the capacity to bring their ideas to
fruition. C. J. Rhodes was the principal agent in
the formation in April 1880 of the De Beers
Mining Company, which rapidly absorbed the re-
maining claims in the mine, and was re-formed in
1887 as tne De Beers Consolidated Mining Com-
pany. Meantime, Barnett Isaacs, better known by
the cognomen Barnato, which had been adopted by his
10
146 GEM-STONES
brother Henry when engaged in earning his livelihood
in the diamond fields as an entertainer, had secured
the major interests in the Kimberley mine. Rhodes
saw that, for effective working of the two mines by
any system of underground working, they must be
under one management, but to all suggestions of
amalgamation Barnato remained deaf, and at last
Rhodes determined to secure control of the Kim-
berley mine at all costs. The story of the titanic
struggle between these two men forms one of the
epics of finance. Eventually, when shares in the
Kimberley mine had been boomed to an extra-
ordinary height, and the price of diamonds had
fallen as low as i8s. a carat, Barnato gave way, and
in July 1889 the Kimberley mine was absorbed by
the De Beers Company on payment of the enor-
mous sum of £5, 3 3 8,6 50. Shortly afterwards they
undertook the working of the Dutoitspan and the
Bultfontein mines, and in January 1896 they
acquired the Premier or Wesselton mine. The
interests in the Jagersfontein mine were in 1888
united in the New Jagersfontein Mining and Ex-
ploration Company, and the mine is now worked also
by the De Beers Company. Thus, until the develop-
ment of the new Premier mine in the Transvaal, the
De Beers Company practically controlled the diamond
market. The development of this last mine was
begun so recently, and its size is so vast — the
longest diameter being half a mile — that open-cut
working is likely to continue for some years.
Though varying slightly in details, the methods of
working the mines are identical in principle. From
the steeply inclined shaft horizontal galleries are
run diagonally right across the mine, the vertical
PLATE XXI
LOADING THE BLUE GROUND ON THE FLOORS, AND PLOUGHING IT OVER
PLATE XXII
OCCURRENCE OF DIAMOND 147
interval between successive galleries being 40 feet.
From each gallery side galleries are run at right
angles to it and parallel to the working face. The
blue ground is worked systematically backwards
from the working face. The mass is stoped, i.e.
drilled and broken from the bottom upwards, until
only a thin roof is left. As soon as the section is
worked out and the material removed, the roof is
allowed to fall in, and work is begun on the next
section of the same level; at the same time the
first section on the level next below is opened out.
Thus work is simultaneously carried on in several
levels, and a vertical plane would intersect the
working faces in a straight line obliquely inclined to
the vertical direction (Fig. 60). When freshly mined,
the blue ground is hard and compact, but it soon dis-
integrates under atmospheric influence. Indeed, the
yellow ground itself was merely decomposed blue
ground. No immediate attempt is made, therefore,
to retrieve the precious stones. The blue ground is
spread on to the 'floors' (Plate XXI), i.e. spaces of
open veldt which have been cleared of bushes and
inequalities, to the depth of a couple of feet, and
remains there for periods ranging from six months
to two years, depending on the quality of the blue
ground and the amount of rainfall. To hasten
the disintegration the blue ground is frequently
ploughed over and occasionally watered, a remark-
able introduction of agricultural methods into mining
operations. No elaborate patrolling or guarding is
required, because the diamonds are so sparsely,
though regularly, scattered through the mass that
even of the actual workers in the mines but few have
ever seen a stone in the blue ground. When
148 GEM-STONES
sufficiently broken up, it is carted to the washing
and concentrating machines, by means of which the
diamonds and the heavier constituents are separated
from the lighter material.
FlG. 60. — Vertical Section of Diamond Pipe, showing Tunnels and Slopes.
Formerly the diamonds were picked out from the
concentrates by means of the keen eyes of skilled
natives ; but the process has been vastly simplified
and the risk of theft entirely eliminated by the
remarkable discovery made in 1897 by F. Kirsten,
PLATE XXI 11
PLATE XX II'
OCCURRENCE OF DIAMOND 149
of the De Beers Company, that of all the heavy
constituents of the blue ground diamond alone, with
the exception of an occasional corundum and zircon,
which are easily sorted out afterwards, adheres to
grease more readily than to water. In this
ingenious machine, the 'jigger ' or 'greaser' (Plate
XXIII) as it is commonly termed, the concentrates are
washed over a series of galvanized-iron trays, which
are covered with a thick coat of grease. The trays
are slightly inclined downwards, and are kept by
machinery in constant sideways motion backwards
and forwards. So accurate is the working of this
device that few diamonds succeed in getting beyond
the first tray, and none progress as far as the third,
which is added as an additional precaution. The
whole apparatus is securely covered in so that there
is no risk of theft during the operation. The trays
are periodically removed, and the grease is scraped
off and boiled to release the diamonds, the grease
itself being used over again on the trays. This is
the first time in the whole course of extraction from
the mines that the diamonds are actually handled.
The stones are now passed on to the sorters, who
separate them into parcels according to their size,
shape, and quality.
The classification at the mines is first into groups
by the shape: (i) close goods, (2) spotted stones,
(3) rejection cleavage, (4) fine cleavage, (5) light
brown cleavage, (6) ordinary and rejection cleavage,
(7) flats, (8) macles, (9) rubbish, (10) boart. Close
goods are whole crystals which contain no flaws and
can be cut into single stones. Spotted stones, as
their name suggests, contain spots which necessitate
removal, and cleavage includes stones which are so
ISO GEM-STONES
full of flaws that they have to be cleaved or split
into two or more stones. Flats are distorted
octahedra, and macles are twinned octahedra.
Rubbish is material which can be utilized only for
grinding purposes, and boart consists of round dark
stones which are invaluable for rock-drills. These
groups are afterwards graded into the following
subdivisions, depending on increasing depth of
yellowish tint : (a) blue-white, (£) first Cape, (c)
second Cape, (d) first bye, (e) second bye, (/) off-
colour, (g) light yellow, (ft) yellow. It is, however, only
the first group that is so minutely subdivided. After
being purchased, the parcels are split up again some-
what differently for the London market (cf. p. 136),
and the dealers re-arrange the stones according to
the purpose for which they are required. Formerly
a syndicate of London merchants took the whole of
the produce of the Kimberley mines at a previously
arranged price per carat, but at the present time
the diamonds are sold by certain London firms on
commission.
The products of each mine show differences in
either form or colour which enable an expert readily
to recognize their origin. The old diggings by the
Vaal River yielded finer and more colourless stones
than those found in the dry diggings and the mines
underlying them. The South African diamonds,
taken as a whole, are always slightly yellowish or
' off-coloured ' ; the mines are, indeed, remarkable
for the number of fine and large, canary-yellow and
brown, stones produced. The Kimberley mine
yields a fair percentage of white, and a large number
of twinned and yellow stones. The yield of the De
Beers mine comprises mostly tinted stones — yellow
OCCURRENCE OF DIAMOND 151
and brown, occasionally silver capes, and very
seldom stones free from colour. The Dutoitspan
mine is noted for its harvest of large yellow
diamonds ; it also produces fine white cleavage and
small white octahedra. The stones found in the
Bultfontein mine are small and spotted, but, on the
other hand, the yield has been unusually regular.
The Premier or Wesselton mine yields a large pro-
portion of flawless octahedra, but, above all, a large
number of beautiful deep-orange diamonds. Of all
the South African mines the Jagersfontein in the
Orange River Colony alone supplies stones of
the highly-prized blue-white colour and steely lustre
characteristic of the old Indian stones. The new
Premier mine in the Transvaal is prolific, but mostly
in off-coloured and low-grade stones, the Cullinan
diamond being a remarkable exception.
To illustrate the amazing productiveness of the
South African mines, it may be mentioned that,
according to Gardner F. Williams, the Kimberley
group of mines in sixteen years yielded 36 million
carats of diamonds, and the annual output of the
Jagersfontein mine averages about a quarter of a
million carats, whereas the total output of the Brazil
mines, for the whole of the long period during which
they have been worked, barely exceeds 13 million
carats. The average yield of the South African
mines, however, perceptibly diminishes as the depth
of the mines increases.
The most interesting point connected with the
South African diamond mines, viewed from the
scientific standpoint, is the light that they have
thrown on the question of the origin of the diamond,
which previously was an incomprehensible and
1 5 2 GEM-STONES
apparently insoluble problem. In the older mines,
just as at the river diggings by the Vaal, the stones
are found in a gravelly deposit that has resulted
from the disintegration of the rocks through which
the adjacent river has passed, and it is clear that
the diamond cannot have been formed in situ here ;
it had been suspected, and now there is no doubt,
that the itacolumite rock of Brazil has consolidated
round the diamonds which are scattered through it,
and that it cannot be the parent rock. The
occurrence at Kimberley is very different. These
mines are funnels which go downwards to unknown
depths; they are more or less oval in section,
becoming narrower with increasing depth, and are
evidently the result of some eruptive agency. The
Kimberley mine has been worked to a depth of
nearly 4000 feet (1200 m.), and no signs of a
termination have as yet appeared. The blue ground
which fills these ' pipes,' as they are termed, must
have been forced up from below, since it is sharply
differentiated from the surrounding country rocks.
This blue ground is a brecciated peridotite of peculiar
constitution, to which the well-known petrologist,
Carvil Lewis, who made a careful study of it, gave
the name kimberlite. The blue colour testifies to its
richness in iron, and it is to the oxidation of the iron
constituent, that the change of colour to yellow in
the upper levels is due. Owing to the shafts that
have been sunk for working the mines, the nature of
the surrounding rocks is known to some depth.
Immediately below the surface is a decomposed
ferriferous basalt, about 20 to 90 feet (6-27 m.)
thick, next a black slaty shale, 200 to 250
feet (60-75 m.) thick, then 10 feet (3 m.) of
OCCURRENCE OF DIAMOND 153
conglomerate, next 400 feet (120 m.) of olivine
diabase, then quartzite, about 400 feet (120 m.)
thick, and lastly a quartz porphyry, which has
not yet been penetrated. The strata run nearly
horizontal, and there are no signs of upward
bending at the pipes. The whole of the country,
including the mines, was covered with a red sandy
soil, and there was nothing to indicate the wealth
that lay underneath. The action of water had in
process of time removed all signs of eruptive activity.
The principal minerals which are associated with
diamond in the blue ground are magnetite, ilmenite,
chromic pyrope, which is put on the market as a
gem under the misnomer ' Cape-ruby,' ferriferous
enstatite, which also is sometimes cut, olivine more
or less decomposed, zircon, kyanite, and mica.
The evidence produced by an examination of the
blue ground and the walls of the pipes proves that
the pipes cannot have been volcanoes such as
Vesuvius. There is no indication whatever of the
action of any excessive temperature, while, on the
other hand, there is every sign of the operation of
enormous pressure; the diamonds often contain
liquid drops of carbonic acid. Crookes puts forward
the plausible theory that steam has been the
primary agency in propelling the diamond and its
associates up into the channel through which it has
carved its way to freedom, and holds that molten
iron has been the solvent for carbon which has
crystallized out as diamond under the enormous
pressures obtaining in remote depths of the earth's
crust. It is pertinent to note that, by dissolving
carbon in molten iron, the eminent chemist, Moissan,
was enabled to manufacture tiny diamond crystals.
154 GEM-STONES
Water trickling down from above would be im-
mediately converted into steam at very high
pressure on coming into contact with the molten
iron, and, in its efforts to escape, the steam would
drive the iron and its precious contents, together
with the adjacent rocks, upwards to the surface.
The ferriferous nature of the blue ground and the
yellow tinge so common to the diamonds lend
confirmation to this theory. The process by which
the carbon was extracted from shales or other
carboniferous rocks and dissolved in iron still awaits
elucidation.
Diamonds were found in New South Wales as long
ago as 1851 on Turon River and at Reedy Creek,
near Bathurst, about ninety miles (145 km.) from
Sydney, but the find was of little commercial import-
ance. A more extensive deposit came to light in
1867 farther north at Mudgee. In 1872 diamonds
were discovered in the extreme north of the State,
at Bingara near the Queensland border. Another
discovery was made in 1884 at Tingha, and still
more recently in the tin gravels of Inverell in the
same region. In their freedom from colour and
absence of twinning the New South Wales diamonds
resemble the Brazilian stones. The average size
is small, running about five to the carat when cut;
the largest found weighed nearly 6 carats when
cut. They are remarkable for their excessive
hardness; they can be cut only with their own
dust, ordinary diamond dust making no impres-
sion.
The Borneo diamonds are likewise distinguished by
their exceptional hardness. They mostly occur by
the river Landak, near Pontianak on the west coast
OCCURRENCE OF DIAMOND 155
of the island. They are found in a layer of rather
coarse gravel, variable, but rarely exceeding a yard
(i m.), in depth, and are associated with corundum
and rutile, together with the precious metals gold
and platinum. Indeed, it is no uncommon sight to
see natives wearing waistcoats ornamented with gold
buttons, in each of which a diamond is set. The
diamonds are well crystallized and generally of
pure water ; yellowish and canary-yellow stones are
also common, but rose-red, bluish, smoky, and
black stones are rare. They seldom exceed a
carat in weight ; but stones of I o carats in weight
are found, and occasionally they attain to 20
carats. In 1850 a diamond weighing 77 carats
was discovered. The Rajah of Mattan is said
to possess one of the purest water weighing as
much as 367 carats, but no one qualified to pro-
nounce an opinion regarding its genuineness has
ever seen it.
In Rhodesia small diamonds have been found
in gravel beds resting on decomposed granite near
the Somabula forest, about 12 miles (19 km.) west
of Gwelo, in association with chrysoberyl in abund-
ance, blue topaz, kyanite, ruby, sapphire, tourmaline,
and garnet.
The occurrence of diamond in German South-
West Africa is very peculiar. Large numbers of
small stones are found close to the shore near
Luderitz Bay in a gravelly surface layer, which is
nowhere more than a foot in depth. They are
picked by hand by natives and washed in sieves.
In shape they are generally six-faced octahedra
or twinned octahedra, simple octahedra being rare,
and in size they run about four or five to the
156 GEM-STONES
carat, the largest stone as yet found being only
2 carats in weight. Their colour is usually
yellowish.
Several isolated finds of diamonds have been
reported in California and other parts of the United
States, but none have proved of any importance.
The largest stone found weighed 23! carats uncut;
it was discovered at Manchester in Virginia.
CHAPTER XVIII
HISTORICAL DIAMONDS
THE number of diamonds which exceed a
hundred carats in weight when cut is very
limited. Their extreme costliness renders them
something more than mere ornaments ; in a
condensed and portable form they represent great
wealth and all the potentiality for good or ill
thereby entailed, and have played no small, if
sinister, r61e in the moulding of history. In bygone
days when despotic government was universal, the
possession of a splendid jewel in weak hands but
too often precipitated the aggression of a greedy
and powerful neighbour, and plunged whole
countries into the horrors of a ruthless and bloody
war. In more civilized days a great diamond has
often been pledged as security for money to
replenish an empty treasury in times of stress.
The ambitions of Napoleon might have received
a set-back but for the funds raised on the security
of the famous Pitt diamond. The history of
such stones — often one long romance — is full of
interest, but space will not permit of more than
a brief sketch here.
If we except the colossal Cullinan stone, the
mines of Brazil and South Africa cannot compare
with the old mines of India as the birthplace of
large and perfect diamonds of world-wide fame.
157
158
GEM-STONES
FIG. 61. — Koh-i-nor (top
view).
(l) KOH-I-NOR
The history of the famous stone called the
Koh-i-nor, meaning Mound of Light, is known as
far back as the year 1304, when it fell into the
hands of the Mogul em-
perors, and legend even
traces it back some four
thousand years previously.
It remained at Delhi until
the invasion of North- West
India by Nadir Shah in
1739, when it passed to-
gether with an immense
amount of spoil into the
hands of the conqueror.
At his death the empire which he had so strenu-
ously founded fell to pieces, and the great diamond
after many vicissitudes came into the possession of
Runjit Singh at Lahore. His successors kept it
until upon the fall of the
Sikh power in 1850 it
passed to the East India
Company, in whose name
it was presented by Lord
Dalhousie to Queen Victoria.
At this date the stone still
retained its original Indian
form, but in 1862 it was re-cut into the form of
a shallow brilliant (Fig. 62), the weight thereby
being reduced from i86TV to io6TV carats. The
wisdom of this course has been severely criticized ;
the stone has not the correct shape of a brilliant
and is deficient in ' fire,' and it has with the change
FIG. 62.— Koh-i-nor (side
view).
HISTORICAL DIAMONDS
159
in shape lost much of its old historical interest.
The Koh-i-nor is the private poperty of the English
Royal Family, the stone shown in the Tower being
a model. It is valued at ,£100,000.
FIG. 63.— Pitt or Regent
(top view).
(2) PITT OR REGENT
This splendid stone was discovered in 1701 at
the famous diamond mines
at Partial, on the Kistna,
about 150 miles (240 km.)
from Golconda, and weighed
as much as 410 carats in the
rough. By devious ways it
came into the hands of Jam-
chund, a Parsee merchant,
from whom it was purchased
by William Pitt, governor of
Fort St. George, Madras, for
£20,400. On his return to England Pitt had it cut
into a perfect brilliant (Fig. 63), weighing 163^
carats, the operation ocupying the space of two
years and costing £5000 ; more
than £7000 is said to have
been realized from the sale of
the fragments left over. Pitt
had an uneasy time and lived
in constant dread of theft of the
Re§ent stone until> in 1 7 1 7> after lensthy
negotiations, he parted with it to
the Due d'Orleans, Regent of France, for the immense
sum of three and three-quarter million francs, about
£,1 3 5,000. With the remainder of the French regalia
it was stolen from the Garde-meuble on August r7,
160 GEM-STONES
1792, in the early days of the French Revolution,
but was eventually restored by the thieves, doubt-
less because of the impossibility of disposing of such
a stone, at least intact, and it is now exhibited
in the Apollo Gallery of the Louvre at Paris. It
measures about 30 millimetres in length, 25 in width,
and 19 in depth, and is valued at ^480,000.
(3) ORLOFF
One of the finest diamonds existing, this large
stone forms the top of
the imperial sceptre of
FIG. 65.— Orloff (top view). FIG. 66.— Orloff (side view).
Russia. It is rose-cut (Fig. 65), the base being a
cleavage face, and weighs i94f carats. It is said
to have formed at one time one of the eyes of a
statue of Brahma which stood in a temple on the
island of Sheringham in the Cavery River, near
Trichinopoli, in Mysore, and to have been stolen
by a French soldier who had somehow persuaded
the priests to appoint him guardian of the temple.
He sold it for £2000 to the captain of an English
ship, who disposed of it to a Jewish dealer in
London for ,£12,000. It changed hands to a
Persian merchant, Raphael Khojeh, who eventually
sold it to Prince Orloff for, so it is said, the immense
HISTORICAL DIAMONDS 161
sum of £90,000 and an annuity of £4000. It
was presented by Prince Orloff to Catherine II of
Russia.
(4) GREAT MOGUL
This, the largest Indian diamond known, was
found in the Kollur mines, about the year 1650.
Its original weight is said to have been 787^ carats,
but it was so full of flaws that the Venetian, Hortensio
Borgis, then in India, in cutting it to a rose form
reduced its weight to 240 carats. It was seen by
Tavernier at the time of his visit to India, but it
has since been quite lost sight of. It has been
identified with both the Koh-i-nor and the Orloff,
and it is even suggested that both these stones were
cut from it.
(5) SANCY
The history of this diamond is very involved, and
probably two or more stones have been confused.
It may have been the one cut by Berquem for
Charles the Bold, from whose body on the fatal day
of Nancy, in 1477, it was snatched by a marauding
soldier. It was acquired by Nicholas Harlai,
Seigneur de Sancy, who sold it to Queen Elizabeth
at the close of the sixteenth century. A hundred
years later, in 1695, it was sold by James II to
Louis XIV. The stone in the French regalia,
according to the inventory taken in 1791, weighed
53! carats. It was never recovered after the theft
of the regalia in the following year, but may be
identical with the diamond which was in the posses-
sion of the Demidoff family and was sold by Prince
Demidoff in 1865 to a London firm who were said
1 62 GEM-STONES
to have been acting for Sir Jamsetjee Jeejeebhoy,
a wealthy Parsee of Bombay. It was shown at the
Paris Exhibition of 1867. It was almond-shaped,
and covered all over with tiny facets by Indian
lapidaries.
(6) GREAT TABLE
This mysterious stone was seen by Tavernier at
Golconda in 1642, but has quite disappeared. It
weighed 242^ carats.
(7) MOON OF THE MOUNTAINS
This diamond is often confused with the Orloff.
It was captured by Nadir Shah at Delhi, and after
his murder was stolen by an Afghan soldier who
disposed of it to an Armenian, by name Shaffrass.
It was finally acquired by the Russian crown for an
enormous sum.
(8) NIZAM
A large diamond, weighing 340 carats, belonged
to the Nizam of Hyderabad ; it was fractured at
the beginning of the Indian Mutiny. Whether the
weight is that previous to fracture or not, there
seems to be no information.
(9) DARYA-I-NOR
This fine diamond, rose-cut and 186 carats in
weight, is of the purest water and merits its title of
' River of Light.' It seems to have been captured
by Nadir Shah at Delhi, and is now the largest
diamond in the Persian collection.
HISTORICAL DIAMONDS 163
(10) SHAH
This fine stone, of the purest water, was pre-
sented to the Czar Nicholas by the Persian prince
Chosroes, younger son of Abbas Mirza, in 1843.
At that time it still retained three cleavage faces
which were engraved with the names of three
Persian sovereigns, and weighed 95 carats. It was,
however, subsequently re-cut with the loss of 9
carats, and the engraving has disappeared in the
process.
(n) AKBAR SHAH, OR JEHAN GHIR SHAH
Once the property of the great Mogul, Akbar, this
diamond was engraved on two faces with Arabic
inscriptions by the instructions of his successor,
Jehan. It disappeared, but turned up again in
Turkey under the name of ' Shepherd's Stone ' ; it
still retained its original inscriptions and was there-
by recognized. In 1866 it was re-cut, the weight
being reduced from 116 to 71 carats, and the in-
scriptions destroyed. The stone was sold to the
Gaekwar of Baroda for 3^ lakhs of rupees (about
£23,333).
(12) POLAR STAR
A beautiful, brilliant-cut stone, weighing 40
carats, which is known by this name, is in the
Russian regalia.
(13) NASSAK
The Nassak diamond, which weighed 89! carats,
formed part of the Deccan booty, and was put up
1 64 GEM-STONES
to auction in London in July 1837. It was pur-
chased by Emanuel, a London jeweller, who for
£7200 shortly afterwards sold it to the Duke of
Westminster, in whose family it still remains. It
was originally pear-shaped, but was re-cut to a
triangular form with a reduction in weight to 78f
carats.
(14) NAPOLEON
This diamond was purchased by Napoleon
Buonaparte for £8000, and worn by him at his
wedding with Josephine Beauharnais in 1796.
(15) CUMBERLAND
This stone, which weighs 32 carats, was purchased
by the city of London for £10,000 and presented to
the Duke of Cumberland after the battle of Culloden ;
it is now in the possession of the Duke of Brunswick.
(16) PlGOTT
A fine Indian stone, weighing 47^ carats, this
diamond was brought to England by Lord Pigott in
1775 and sold for £30,000. It came into the
possession of Ali Pacha, Viceroy of Egypt, and was
by his orders destroyed at his death.
(17) EUGENIE
This fine stone, weighing 5 I carats, was given by
the Czarina Catherine II of Russia to her favourite,
Potemkin. It was purchased by Napoleon in as a
bridal gift for his bride, and on his downfall was
bought by the Gaekwar of Baroda.
HISTORICAL DIAMONDS 165
(18) WHITE SAXON
Square in contour, measuring I ^ in. (28 mm.),
and weighing 48! carats, this stone was purchased
by Augustus the Strong for a million thalers (about
£150,000).
(19) PACHA OF EGYPT
This 4O-carat brilliant was purchased by Ibrahim,
Viceroy of Egypt, for £28,000.
(20) STAR OF ESTE
Though a comparatively small stone, in weight
25^ carats, it is noted for its perfection of form and
quality. It belongs to the Archduke Franz Ferdi-
nand of Austrian-Este, eldest son of the Archduke
Karl Ludwig.
(21) TUSCANY, OR AUSTRIAN YELLOW
The beauty of this large stone, 133! carats in
weight, is marred by the tinge of yellow, which is
sufficiently pronounced to impair its brilliancy ;
it is a double rose in form. At one time the
property of the Grand Dukes of Tuscany, it is now
in the possession of the Emperor of Austria. King
mentions a tale that it was bought at a curiosity
stall in Florence for an insignificant sum, the stone
being supposed to be only yellow quartz.
(22) STAR OF THE SOUTH
This, the largest of the Brazilian diamonds, was
discovered at the mines of Bagagem in July 1853.
1 66 GEM-STONES
Perfectly transparent and without tint, it was
dodecahedral in shape and weighed 254! carats,
and was sold in the rough for £40,000. It was cut
as a perfect brilliant, being reduced in weight to
125^ carats.
(23) ENGLISH DRESDEN
This beautiful stone, which weighed 1 1 9^ carats
in the rough, was found at the Bagagem mines, in
Brazil, in 1857, a°d came into the possession of
Mr. E. Dresden. It was cut as a long, egg-shaped
brilliant, weighing 76^ carats.
(24) STAR OF SOUTH AFRICA
The first considerable stone to be found in South
Africa, it was discovered at the Vaal River diggings
in 1869, and weighed 83^ carats in the rough. It
was cut to a triangular brilliant of 46^ carats.
It was finally purchased by the Countess of Dudley
for £25,000.
(25) STEWART
This large diamond, weighing in the rough 288f
carats, was found at the Vaal River diggings in
1872, and was first sold for £6000 and shortly
afterwards for £9000 ; it was reduced on cutting to
1 20 carats. Like many South African stones, it
has a faint yellowish tinge.
(26) PORTER-RHODES
This blue- white stone, which weighed 150 carats,
was found in a claim belonging to Mr. Porter-
Rhodes in the Kimberley mine in February 1880.
HISTORICAL DIAMONDS 167
(27) IMPERIAL, VICTORIA, OR GREAT WHITE
This large diamond weighed as much as 457
carats in the rough, and 180 when cut; it is quite
colourless. It was brought to Europe in 1884, and
was eventually sold to the Nizam of Hyderabad
for £20,000.
(28) DE BEERS
A pale yellowish stone, weighing 42 8 \ carats,
was found in the De Beers mine in 1888. It was
cut to a brilliant weighing 22 8 1 carats, and was sold
to an Indian prince. A still larger stone of similar
tinge, weighing 503^ carats, was discovered in 1896,
and among other large stones supplied by the same
mine may be mentioned one of 302 carats found in
1 884, and another of 409 carats found in early years.
(29) EXCELSIOR
This, which prior to the discovery of the ' Cullinan,'
was by far the largest South African stone, was
found in the Jagersfontein mine on June 30, 1893 j
bluish-white in tint, it weighed 969^ carats. From
it were cut twenty-one brilliants, the larger stones
weighing 67!, 45U, 45H, 39&, 34, 27$, 25!, 23^,
I &r|, 1 3 £ carats respectively, and the total weight
of the cut stones amounting to 364^% carats.
(30) JUBILEE
Another large stone was discovered in the
Jagersfontein mine in 1895. It weighed 634
carats in the rough, and from it was obtained a
splendid, faultless brilliant weighing 239 carats. It
was shown at the Paris Exhibition of 1900,
i68
GEM-STONES
(31) STAR OF AFRICA, OR CULLINAN
All diamonds pale into insignificance when com-
pared with the colossal stone that came to light at
the Premier mine near Pretoria in the Transvaal on
January 25, 1905. It was first called the
' Cullinan ' after Sir T. M. Cullinan, chairman of the
Premier Diamond Mine (Transvaal) Company, but
has recently, by desire
of King George V, re-
ceived the name ' Star
of Africa.' The rough
stone weighed 621-2
grams or 3025!- carats
(about i£ lb.); it dis-
played three natural
faces (Plate XXV) and
one large cleavage face,
and its shape suggested
that it was a portion
of an enormous stone
more than double its
size ; it was trans-
parent, colourless, and
had only one small flaw near the surface. This
magnificent diamond was purchased by the Trans-
vaal Government for £150,000, and presented to
King Edward VII on his birthday, November 9, 1907.
The Cullinan was entrusted to the famous firm,
Messrs. I. J. Asscher & Co., of Amsterdam, for
cutting on January 23, 1908, just three years after
its discovery. On February 10 it was cleaved into
two parts, weighing respectively 1977^ and 1040^
carats, from which the two largest stones have been
FIG. 67.— Cullinan No.
PLATE XXV
HISTORICAL DIAMONDS
169
cut, one being a pendeloque or drop brilliant in
shape (Fig. 67) and weighing 5 1 6% carats, and the
other a square brilliant (Fig. 68) weighing 3O9TV
carats. The first has been placed in the sceptre, and
the second in the
crown of the regalia.
Besides these there
are a pendeloque
weighing 92 carats, a
square-shaped brilli-
ant 62, a heart-shaped
stone 1 8 1 , two mar-
quises 8TV and 1 1 J,
an oblong stone 6|,
a pendeloque 4-^-,
and 96 small brill-
iants weighing to-
gether 7 1 ; the total weight of the cut stones
amounts to 1036/0 carats. The largest stone has
74 and the second 66 facets. The work was
completed and the stones handed to King Edward
in November 1908.
Although the Premier mine has yielded no worthy
compeer of the Cullinan, it can, nevertheless, boast
of a considerable number of large stones which but
for comparison with that giant would be thought
remarkable for their size, no fewer than seven of
them having weights of over 300 carats, viz. 511,
, 45 8f, 39ii, 373. 348, and 334 carats.
FIG. 68.— Cullinan No. 2.
(32) STAR OF MINAS
This large diamond, which was found in 1911 at
the Bagagem mines, Minas Geraes, Brazil, had the
1 70 GEM-STONES
shape of a dome with a flat base, and weighed in
the rough 3 5 '8 7 5 grams (174! carats).
The large stone called the ' Braganza,' in the
Portuguese regalia, which is supposed to be a
diamond, is probably a white topaz ; it weighs
1680 carats. The Mattan stone, pear-shaped and
weighing 367 carats, which was found in the Landak
mines near the west coast ot Borneo in 1787, is
suspected to be quartz.
COLOURED DIAMONDS
(i) HOPE
The largest of coloured diamonds, the Hope,
weighs 44^ carats, and has a steely- or greenish-
blue, and not the royal-blue colour of the glass
models supposed to represent it. It is believed to
be a portion of a drop- form stone
(d'un beau violet} which was said
to have been found at the Kollur
mines, and was secured by Taver-
nier in India in 1642 and sold
by him to Louis XIV in 1668;
FIG. 69.-Hope. * then weighed 67 carats. This
stone was stolen with the re-
mainder of the French regalia in 1792 and never
recovered. In 1830 the present stone (Fig. 69)
was offered for sale by Eliason, a London dealer,
and was purchased for £18,000 by Thomas Philip
Hope, a wealthy banker and a keen collector of
gems. Probably the apex of the original stone
had been cut off, reducing it to a nearly square
HISTORICAL DIAMONDS 171
stone. The slight want of symmetry of the present
stone lends confirmation to this view, and two other
blue stones are known, which, together with 'the
Hope, make up the weight of the original stone.
At the sale of the Hope collection at Christie's in
1867 the blue diamond went to America. In 1908
the owner disposed of it to Habib Bey for the
enormous sum of £80,000. It was put up to
auction in Paris in 1909, and bought by Rosenau,
the Paris diamond merchant, for the comparatively
small sum of 400,000 francs (about £16,000), and
was sold in January 1911 to Mr. Edward M'Lean
for £60,000. The stone is supposed to bring ill-
luck in its train, and its history has been liberally
embellished with fable to establish the saying.
(2) DRESDEN
A beautiful apple-green diamond, faultless, and
of the purest water, is contained in the famous
Green Vaults of Dresden. It weighs 40 carats, and
was purchased by Augustus the Strong in 1743 for
60,000 thalers (about £9000).
(3) PAUL I
A fine ruby-red diamond, weighing 10 carats, is
included among the Russian crown jewels.
(4) TIFFANY
The lovely orange brffliant, weighing 125! carats,
which is in the possession of Messrs. Tiffany & Co.,
the well-known jewellers of New York, was dis-
covered in the Kimberley mine in 1878.
CHAPTER XIX
CORUNDUM
(Sapphire, Ruby}
RANKING in hardness second to diamond
alone, the species known to science as
corundum and widely familiar by the names of its
varieties, sapphire and ruby, holds a pre-eminent
position among coloured gem-stones. The barbaric
splendour of ruby (Plate I, Fig. 13) and the
glorious hue of sapphire (Plate I, Fig. n) are
unsurpassed, and it is remarkable that the same
species should boast such different, but equally
magnificent, tints. They, however, by no means
exhaust the resources of this variegated species.
Fine yellow stones (Plate I, Fig. 12), which compare
with topaz in colour and are its superior in hard-
ness, and brilliant colourless stones, which are
unfortunately deficient in ' fire ' and cannot there-
fore approach diamond, are to be met with, besides
others of less attractive hues, purple, and yellowish,
bluish, and other shades of green. Want of homo-
geneity in the coloration of corundum is a frequent
phenomenon ; thus, the purple stones on close
examination are found to be composed of alternate
blue and red layers, and stones showing patches of
yellow and blue colour are common. Owing to the
173
CORUNDUM 173
peculiarity of their interior arrangement certain
stones display when cut en cabochon a vivid six-
rayed star of light (Plate I, Fig. 15). Sapphire
and ruby share with diamond, pearl, and emerald
the first rank in jewellery. They are popular stones,
especially in rings ; their comparative rarity in large
sizes, apart from the question of expense, prevents
their use in the bigger articles of jewellery. The
front of the stones is usually brilliant-cut and the
back step-cut, but Indian lapidaries often prefer to
cover the stone with a large number of triangular
facets, especially if the stone be flawed ; star-stones
are cut more or less steeply en cabochon.
In composition corundum is alumina, oxide of
aluminium, corresponding to the formula A12O3, but
it usually contains in addition small quantities,
rarely more than I per cent., of ferric oxide, chromic
oxide, and perhaps other metallic oxides. When
pure, it is colourless ; the splendid tints which are
its glory have their origin in the minute traces of
the other oxides present. No doubt chromic oxide
is the cause of the ruddy hue of ruby, since it is
possible, as explained above (p. 117), closely to
imitate the ruby tint by this means, but nothing
approaching so large a percentage as 2\ has been
detected in a natural stone. The blue colour of
sapphire may be due to titanic oxide, and ferric
oxide may be responsible for the yellow hue of the
' oriental topaz,' as the yellow corundum is termed.
Sapphires, when of considerable size, are rarely
uniform in tint throughout the stone. Alternations
of blue and red zones, giving rise to an apparent
purple or violet tint, and the conjunction of patches
of blue and yellow are common. Perfectly colour-
174 GEM-STONES
less stones are less common, a slight bluish tinge
being usually noticeable, but they are not in much
demand because, on account of their lack of ' fire,'
they are of little interest when cut. The tint of the
red stones varies considerably in depth ; jewellers
term them, when pale, pink sapphires, but, of course,
no sharp distinction can be drawn between them and
rubies. The most highly prized tint is the so-called
pigeon's blood, a shade of red slightly inclined to
purple. The prices for ruby of good colour run
from about 253. a carat for small stones to between
£,60 and £80 a carat for large stones, and still
higher for exceptional rubies. The taste in sapphires
has changed of recent times. Formerly the deep
blue was most in demand, but now the lighter shade,
that resembling the colour of corn-flower, is preferred,
because it retains a good colour in artificial light.
Large sapphires are more plentiful than large rubies,
and prices run lower ; even for large perfect stones
the rate does not exceed £30 a carat. Large and
uniform ' oriental topazes ' are comparatively
common, and realize moderate prices, about 2s. to
305. a carat according to quality and size. Green
sapphires are abundant from Australia, but their
tint, a kind of deep sage-green, is not very pleasing.
Brown stones with a silkiness of structure are also
known.
The name of the species comes through the
French corindon from an old Hindu word, korund,
of unknown significance, and arose from the circum-
stance that the stones which first found their way to
Europe came from India. At the present day the
word corundum is applied in commerce to the
opaque stones used for abrasive purposes, to 'dis-
CORUNDUM 175
tinguish the purer material from emery, which is
corundum mixed with magnetite and other heavy
stones of lower hardness. The origin of the word
sapphire, which means blue, has been discussed in an
earlier chapter (p. 1 1 o). Jewellers use it in a
general sense for all corundum except ruby. Ruby
comes from the Latin ruber, red. The prefix
' oriental ' (p. in) is often used to distinguish
varieties of corundum, since it is the hardest of
ordinary coloured stones and the finest gem-stones
in early days reached Europe by way of the
East.
Corundum crystallizes either in six-sided prisms
terminated by flat faces (Plate I, Fig. 10), which
are often triangularly marked, or with twelve inclined
faces, six above and six below, meeting in a girdle
(Plate I, Fig. 14). Ruby favours the former and
the other varieties the latter type. A fine crystal of
ruby — the ' Edwardes/ so named by the donor, John
Ruskin, after Sir Herbert Edwardes — which weighs
33-5 grams (163 carats), is exhibited in the Mineral
Gallery of the British Museum (Natural History),
and is tilted in such a way that the light from a
neighbouring window falls on the large basal face,
and reveals the interesting markings that nature has
engraved on it. From its type of symmetry corundum
is doubly refractive with a direction of single refraction
running parallel to the edge of the prism. Owing to
the relative purity of the chemical composition the
refractive indices are very constant ; the ordinary
index ranges from 1766 to 1774 and the extra-
ordinary index from 1757 to 1765, the double
refraction remaining always the same, 0*009. 1 he
amount of colour-dispersion is small, and therefore
i;6 GEM-STONES
colourless corundum displays very little ' fire.' The
difference between the indices for red and blue light
is, however, sufficiently great that the base of a
ruby may be left relatively thicker than that of a
sapphire to secure an equally satisfactory effect
(cf. p. 98) — a point of some importance to the
lapidary, since stones are sold by weight and it is
his object to keep the weight as great as possible.
When a corundum is tested on the refractometer in
white light a wide spectrum deliminates the two
portions of the field because of the smallness of the
colour-dispersion (cf. p. 25). The dichroism of both
ruby and sapphire is marked, the twin colours given
by the former being red and purplish-red, and by
the latter blue and yellowish-blue, the second colour
in each instance corresponding to the extraordinary
ray. Tests with the dichroscope easily separate
ruby and sapphire from any other red or blue stone.
This character has an important bearing on the
proper mode of cutting the stones. The ugly
yellowish tint given by the extraordinary ray of
sapphire should be avoided by cutting the stone
with its table-facet at right angles to the prism edge,
which is the direction of single refraction. Whether
a ruby should be treated in the same way is a moot
point. No doubt if the colour is deep, it is the best
plan, because the amount of absorption of light is
thereby sensibly reduced, but otherwise the delightful
nuances distinguishing ruby are best secured by
cutting the table-facet parallel to the direction of
single refraction. Yellow corundum also shows
distinct dichroism, but by a variation more of the
depth than of the tint of the colour ; the phenomenon
is faint compared with the dichroic effect of a yellow
CORUNDUM 177
chrysoberyl. The specific gravity also is very
constant, varying only from 3-95 to 4*10; sapphire
is on the whole lighter than ruby. Corundum has
the symbol 9 on Mohs's scale, but though coming
next to diamond it is a very poor second (cf. p. 79).
As is usually the case, the application of heat tends
to lighten the colour of the stones : those of a pale
violet or a yellow colour lose the tint entirely, and
the deep violet stones turn a lovely rose colour. On
the other hand the action of radium has, as was
shown by Bordas, an intensifying action on the
colour, and even develops it in a colourless stone.
From the latter reaction it may be inferred that often
in an apparently colourless stone two or more
selective influences are at work which ordinarily
neutralize one another,but, being unequally stimulated
by the action of radium, they thereupon give rise to
colour. The stellate appearance of asterias or star-
stones — star-ruby and star-sapphire — results from
the regular arrangement either of numerous small
channels or of twin-lamellae in the stone parallel to
the six sides of the prisms ; light is reflected from
the interior in the form of a six-rayed star (p. 38).
Some stones from Siam possess a markedly fibrous
or silky structure.
The synthetical manufacture of ruby, sapphire,
and other varieties of corundum has already been
described (p. 1 1 6).
Besides its use in jewellery corundum is on ac-
count of its hardness of great service for many other
purposes. Small fragments are extensively employed
for the bearing parts of the movements of watches,
and both the opaque corundum and the impure
kind known as emery are in general use for
12
1 78 GEM-STONES
grinding and polishing softer stones, and steel and
other metal-work.
The world's supply of fine rubies is drawn almost
entirely from the famous ruby mines near Mogok,
situated about 90 miles (145 km.) in a north-
easterly direction from Mandalay in Upper Burma
and at a'n elevation of about 4000 ft. (1200 m.)
above sea-level. It is from this district that the
stones of the coveted carmine-red, the so-called
1 pigeon's blood,' colour are obtained. The ruby
occurs in a granular limestone or calcite in associa-
tion with the spinel of nearly the same appearance
— the ' balas-ruby,' oriental topaz (yellow cor-
undum), tourmaline, and occasionally sapphire.
Some stones are found in the limestone on the
sides of the hills, but by far the largest quantity
occur in thek alluvial deposits, both gravel and clay,
in the river-beds ; the ruby ground is locally
known as ' byon' The stones are as a rule quite
small, averaging only about four to the carat.
Before the British annexation of the country in
1885 the mines were a monopoly of the Burmese
sovereigns and were worked solely under royal
licence. They are known to be of great antiquity,
but otherwise their early history is a mystery. It
is said that an astute king secured the priceless
territory in 1597 from the neighbouring Chinese
Shans in exchange for a small and unimportant
town on the Irrawaddy ; if that be so, he struck an
excellent bargain. The mines were allotted to
licensed miners, twin-tsas (eaters of the mine) as
they were called in the language of the country,
who not only paid for the privilege, but were
compelled to hand over to the king all stones
CORUNDUM 179
above a certain weight As might be anticipated
this injunction caused considerable trouble, and
the royal monopolists constantly suspected the miners
of evading the regulation by breaking up stones
of exceptional size; from subsequent experience,
it is probable that large stones were in reality
seldom found. Since 1887 the mines have been
worked by arrangement with the Government of
India by the Ruby Mines, Ltd., an English
company. Its career has been far from prosper-
ous, but during recent years, in consequence of
the improved methods of working the mines and
of the more generous terms afterwards accorded
by the Government, greater success has been
experienced ; the future is, however, to some extent
clouded by the advent of the synthetical stone, which
has even made its way out to the East.
Large rubies are far from common, and such
as were discovered in the old days were jealously
hoarded by the Burmese sovereigns. According
to Streeter the finest that ever came to Europe were
a pair brought over in 1875, at a time when the
Burmese king was pressed for money. One, rich
in colour, was originally cushion-shaped and weighed
37 carats; the other was a blunt drop in form
and weighed 47 carats. Both were cut in London,
the former being reduced to 32^ carats and the
latter to 38 A carats, and were sold for £10,000
and £20,000 respectively. A colossal stone,
weighing 400 carats, is reported to have been found
in Burma ; it was broken into three pieces, of
which two were cut and resulted in stones weighing
70 and 45 carats respectively, and the third was
sold uncut in Calcutta for 7 lakhs of rupee?
i8o GEM-STONES
(£46,667). The finder of another large stone
broke it into two parts, which after cutting weighed
98 and 74 carats respectively ; he attemped in
vain to evade the royal acquisitiveness, by giving
up the larger stone to the king and concealing
the other. A fine stone, known by the formidable
appellation of ' Gnaga Boh ' (Dragon Lord),
weighed 44 carats in the rough and 20 carats
after cutting. Since the mines were taken over
by the Ruby Mines, Ltd., a few large stones have
been discovered. A beautiful ruby was found in
the Tagoungnandaing Valley, and weighed i8|
carats in the rough and 1 1 carats after cutting ;
perfectly clear and of splendid colour, it was sold for
£7000, but is now valued at £10,000. Another,
weighing 77 carats in the rough, was found in
1899, and was sold in India in 1904 for 4 lakhs
of rupees (£26,667). A stone, weighing 49 carats,
was discovered in 1887, and an enormous one,
weighing as much as 304 carats, in 1890.
The ruby, as large as a pigeon's egg, which is
amongst the Russian regalia was presented in
1777 to the Czarina Catherine by Gustav III of
Sweden when on a visit to St. Petersburg. The
large red stone in the English regalia which was
supposed to be a ruby is a spinel (cf. p. 206).
Comparatively uncommon as sapphires are in
the Burma mines a faultless stone, weighing as
much as 79^ carats, has been discovered there.
Good rubies, mostly darker in colour than the
Burmese stones, are found in considerable quantity
near Bangkok in Siam, Chantabun being the centre
of the trade, where, just as in Burma, they are
intimately associated with the red spinel. Because
CORUNDUM 1 8 1
of the difference in tint and the consequent
difference in price, jewellers draw a distinction
between Burma and Siam rubies ; but that, of
course, does not signify any specific difference
between them. Siam is, however, most distin-
guished as the original home of splendid sapphires.
Th£ district of Bo Pie Rin in Battambang produces,
indeed, more than half the world's supply of
sapphires. In the Hills of Precious Stones, such
being the meaning of the native name for the locality,
a number of green corundums are found. Siam
also produces brown stones characterized by a
peculiar silkiness of structure. Rubies are found
in Afghanistan at the Amir's mines near Kabul
and also to the north of the lapis lazuli mines
in Badakshan.
The conditions in Ceylon are precisely the
converse of those obtaining in Burma; sapphire is
plentiful and ruby rare in the island. They are
found in different rocks, sapphire occurring with
garnet in gneiss, and ruby accompanying spinel in
limestone, but they come together in the resulting
gravels, the principal locality being the gem-district
near Ratnapura in the south of the island. The
largest uncut ruby discovered in Ceylon weighed
42^ carats; it had, however, a decided tinge of blue
in it. Ceylon is also noted for the magnificent
yellow corundum, ' oriental topaz,' or, as it is
locally called, ' king topaz,' which it produces.
Beautiful sapphires occur in various parts of
India, but particularly in the Zanskar range of the
north-western Himalayas in the state of Kashmir,
where they are associated with brown tourmaline.
Probably most of the large sapphires known have
1 82 GEM-STONES
emanated from India. By far the most gigantic
ever reported is one, weighing 951 carats, said to
have been seen in 1827 in the treasury of the
King of Ava. The collection at the Jardin des
Plantes contains two splendid rough specimens ;
one, known as the ' Rospoli,' is quite flawless
and weighs I32TV carats, and the other is 2 inches
in length and i^ inches in thickness. The Duke
of Devonshire possesses a fine cut stone, weighing
100 carats, which is brilliant-cut above and step-
cut below the girdle. An image of Buddha, which
is cut out of a single sapphire, is exhibited,
mounted on a gold pin, in the Mineral Gallery of
the British Museum (Natural History).
For some years past a large quantity of sap-
phires have come into the market from Montana,
U.S.A., especially from the gem-district about
twelve miles west of Helena. The commonest
colour is a bluish green, generally pale, but blue,
green, yellow and occasionally red stones are also
found ; they are characterized by their almost met-
allic lustre. With them are associated gold, colour-
less topaz, kyanite, and a beautiful red garnet which
is found in grains and usually mistaken for ruby.
Rubies are also found in limestone at Cowee Creek,
North Carolina.
Blue and red corundum, of rather poor quality,
has come from the Sanarka River, near Troitsk,
and from Miask, in the Government of Orenburg,
Russia, and similar stones have been known at
Campolongo, St. Gothard, Switzerland.
The prolific gem-district near Anakie, Queens-
land, supplies examples of every known variety
of corundum except ruby ; blue, green, yellow,
CORUNDUM 183
and parti-coloured stones, and also star-stones,
are plentiful. Leaf-green corundum is known
farther south, in Victoria. The Australian sapphire
is too dark to be of much value.
Small rubies and sapphires are found in the
gem-gravels near the Somabula Forest, Rhodesia.
CHAPTER XX
BERYL
(Emerald, Aquamarine, Morganite)
THE species to be considered in this chapter
includes the varieties emerald and aquamarine,
as well as what jewellers understand by beryl. It
has many incontestable claims on the attention of
all lovers of the beautiful in precious stones. The
peerless emerald (Plate I, Fig. 5), which in its ver-
dant beauty recalls the exquisite lawns that grace
the courts and quadrangles of our older seats of
learning, ranks to-day as the most costly of jewels.
Its sister stone, the lovely aquamarine (Plate I,
Fig. 4), which seems to have come direct from some
mermaid's treasure-house in the depths of a summer
sea, has charms not to be denied. Pliny, speaking
of this species, truly says, " There is not a colour
more pleasing to the eye " ; yet he knew only the
comparatively inferior stones from Egypt, and
possibly from the Ural Mountains. Emeralds are
favourite ring-stones, and would, no doubt, be equally
coveted for larger articles of jewellery did not the
excessive cost forbid, and nothing could be more
attractive for a central stone than a choice aqua-
marine of deep blue- green hue. Emeralds are
usually step-cut, though Indian lapidaries often
184
BERYL 185
favour the en cabochon form ; aquamarines, on the
other hand, are brilliant-cut in front and step-cut at
the back.
Beryl, to use the name by which the species is
known to science, is essentially a silicate of aluminium
and beryllium corresponding to the formula, Be3Al2
(SiO3)6. The beryllia is often partially replaced by
small amounts of the alkaline earths, caesia, potash,
soda, and lithia, varying from about i| per cent, in
beryl from Mesa Grande to nearly 5 in that from
Pala and Madagascar, and over 6, of which 3-6 is
caesia, in beryl from Hebron, Maine ; also, as usual,
chromic and ferric oxides take the place of a little
alumina ; from I to 2 per cent, of water has been
found in emerald. The element beryllium was, as
its name suggests, first discovered in a specimen of
this species, the discovery being made in 1798 by
the chemist Vauquelin ; it is also known as glucinum
in allusion to the sweet taste of its salts.
When pure, beryl is colourless, but it is rarely, if
ever, free from a tinge of blue or green. The colour
is usually some shade of green — grass-green, of that
characteristic tint which is in consequence known as
emerald-green, or blue-green, yellowish green (Plate
I, Fig. 6), and sometimes yellow, pink, and rose-
red. The peculiar colour of emerald is supposed to
be caused by chromic oxide, small quantities of
which have been detected in it by chemical analysis ;
moreover, experiment shows that glass containing
the same percentage amount of chromic oxide
assumes the same splendid hue. Emerald, on being
heated, loses water, but retains its colour unimpaired,
which cannot therefore be due, as has been suggested,
to organic matter. The term aquamarine is applied
1 86 GEM-STONES
to the deep sea-green and blue-green stones, and
jewellers restrict the term beryl to paler shades and
generally other colours, such as yellow, golden, and
pink, but Kunz has recently proposed the name
morganite to distinguish the beautiful rose beryl such
as is found in Madagascar. The varying shades of
aquamarine are due to the influence of the alkaline
earths modified by the presence of ferric oxide or
chromic oxide ; the beautiful blushing hue of mor-
ganite is no doubt caused by lithia.
The name of the species is derived from the Greek
/3?7/3iA\os, an ancient word, the meaning of which
has been lost in the mists of time.
The Greek word denoted the same
species in part as that now under-
stood by the name. Emerald is
derived from a Persian word which
appeared in Greek as (r/jbdpaySos, and
in Latin as smaragdus\ it originally
denoted chrysocolla, or similar green
stone, but was transferred upon the introduction of
the deep-green beryl from Upper Egypt. The name
aquamarine was suggested by Pliny's exceedingly
happy description of the stones " which imitate the
greenness of the clear sea," although it was not actu-
ally used by him. That emerald and beryl were one
species was suspected by Pliny, but the identity was
not definitely established till about a century ago.
Morganite is named after John Pierpont Morgan.
The natural crystals have the form of a six-sided
prism, and in the case of emerald (Fig. 70, and
Plate I, Fig. 8) invariably, if whole, end in a
single face at right angles to the length of the
prism ; aquamarines have in addition a number of
BERYL 187
small inclined faces, and stones from both Russia
and Brazil often taper owing to the effects of
corrosion. The sixfold character of the crystalline
symmetry necessarily entails that the double
refraction, which is small in amount, 0*006,
is uniaxial in character, and, since the ordinary
is greater than the extraordinary refractive
index, it is negative in sign. The values of
the indices range between 1*567 and 1*590, and
1-572 and I -598 respectively, in the two cases, the
pink beryl possessing the highest values. The
dichroism is distinct in the South American emerald,
the twin colours being yellowish and bluish green,
but otherwise is rather faint. The specific gravity
varies between 2*69 and 279, and is therefore a
little higher than that of quartz. If, therefore, a
beryl and a quartz be floating together in a tube
containing a suitable heavy liquid, the former will
always be at a sensibly lower level (cf. Fig. 32).
The hardness varies from 7| to 8, emerald being a
little softer than the other varieties. There is no
cleavage, but like most gem-stones beryl is very
brittle, and can easily be fractured. Stones rendered
cloudy by fissures are termed ' mossy.' When
heated before the blowpipe beryl is fusible with
difficulty ; it resists the attack of hydrofluoric acid
as well as of ordinary acids.
In all probability the whole of the emeralds
known in ancient times came from the so-called
Cleopatra emerald mines in Upper Egypt. For
some reason they were abandoned, and their position
was so completely lost that in the Middle Ages it
was maintained that emeralds had never been found
in Egypt at all, but had come from America by way
1 88 GEM-STONES
of the East. All doubts were set at rest by the
re-discovery of the mines early last century by
Cailliaud, who had been sent by the Viceroy of
Egypt to search for them. They were, however,
not much worked, and after a few years were closed
again, and were re-opened only about ten years ago.
The principal mines are at Jebel Zabara and at
Jebel Sikait in northern Etbai, about 10 miles
(16 km.) apart and distant about 15 miles (24 km.)
from the Red Sea, lying in the range of mountains
that run for a long distance parallel to the west
coast of the Red Sea and rise to over 1800 feet
(550 m.) above sea-level. There are numerous signs
of considerable, but primitive, workings at distinct
periods. Both emeralds and beryls are found in
micaceous and talcose schists. The emeralds are
not of very good quality, being cloudy and rather
light in colour. Finer emeralds have been found in
a dark mica-schist, together with other beryllium
minerals, chrysoberyl and phenakite, and also topaz
and tourmaline on the Asiatic side of the Ural
Mountains, near the Takowaja River, which flows into
the Bolshoi Reft River, one of the larger tributaries of
the Pyschma River, about fifty miles (80 km.) east of
Ekaterinburg, a town which is chiefly concerned with
the mining and cutting of gem-stones. The mine
was accidentally discovered by a peasant, who noticed
a few green stones at the foot of an uprooted tree in
1830. Two years later the mine was regularly
worked, and remained open for twenty years, when
it was closed. It has recently been re-opened
owing to the high rates obtaining for emeralds.
Very large crystals have been produced here, but in
colour they are much inferior to the South American
BERYL 189
stones ; small Siberian emeralds, on the other hand,
are of better colour than small South American
emeralds, the latter being not so deep in tint.
Emeralds have been found in a similar kind of schist
at Habachtal, in the Salzburg Alps. About thirty
years ago well-formed green stones were discovered
with hiddenite at Stony Point, Alexander County,
in North Carolina, but not much gem material has
come to light
The products of none of the mines that have just
been mentioned can on the whole compare with the
beautiful stones which have come from South
America. At the time when the Spaniards grimly
conquered Peru and ruthlessly despoiled the country
of the treasures which could be carried away,
immense numbers of emeralds — some of almost
incredible size — were literally poured into Spain,
and eventually found their way to other parts of
Europe. These stones were known as Spanish or
Peruvian emeralds, but in all probability none of
them were actually mined in Peru. Perhaps the
most extraordinary were the five choice stones which
Cortez presented to his bride, the niece of the Duke
de Bejar, thereby mortally offending the Queen, who
had desired them for herself, and which were lost in
i 529 when Cortez was shipwrecked on his disastrous
voyage to assist Charles V at the siege of Algiers.
All five stones had been worked to divers fantastic
shapes. One was cut like a bell with a fine pearl
for a tongue, and bore on the rim, in Spanish,
" Blessed is he who created thee." A second was
shaped like a rose, and a third like a horn. A
fourth was fashioned like a fish, with eyes of gold.
The fifth, which was the most valuable and the most
190 GEM-STONES
remarkable of all, was hollowed out into the form of
a cup, and had a foot of gold ; its rim, which was
formed of the same precious metal, was engraved
with the words, " Inter natos mulierum non surrexit
major." As soon as the Spaniards had seized nearly
all the emeralds that the natives had amassed in
their temples or for personal adornment, they de-
voted their attention to searching for the source of
these marvels of nature, and eventually in 1558
they lighted by accident upon the mines in what is
now the United States of Colombia, which have been
worked almost continuously since that time. Since
the natives, who naturally resented the gross injustice
with which they had been treated, and penetrated
the greed that prompted the actions of the Spaniards,
hid all traces of the mines, and refused to give any
information as to their position, it is possible that
other emerald mines may yet be found. The
present mines are situated near the village of Muzo,
about 75 miles (120 km.) north-north-west of
Bogota, the capital of Colombia. The emeralds
occur in calcite veins in a bituminous limestone of
Cretaceous age. The Spaniards formerly worked
the mines by driving adits through the barren rock
on the hillsides to the gem-bearing veins, but at the
present day the open cut method of working is
employed. A plentiful supply of water is available,
which is accumulated in reservoirs and allowed at
the proper time to sweep the debris of barren rock
away into the Rio Minero, leaving the rock contain-
ing the emeralds exposed. Stones, of good quality,
which are suited for cutting, are locally known as
canutillos, inferior stones, coarse or ill-shaped, being
called morallons.
BERYL 191
Emerald, unlike some green stones, retains its
purity of colour in artificial light ; in fact, to quote
the words of Pliny, " For neither sun nor shade, nor
yet the light of candle, causeth to change and lose
their lustre." Many are the superstitions that have
been attached to it. Thus it was supposed to be
good for the eyes, and as Pliny says, " Besides, there
is not a gem or precious stone that so fully
possesseth the eye, and yet never contenteth it with
satiety. Nay, if the sight hath been wearied and
dimmed by intentive poring upon anything else, the
beholding of this stone doth refresh and restore it
again." The idea that it was fatal to the eyesight
of serpents appears in Moore's lines —
"Blinded like serpents when they gaze
Upon the emerald's virgin blaze."
The crystals occur attached to the limestone, and
are therefore never found doubly terminated. The
crystal form is very simple, merely a hexagonal
prism with a flat face at the one end at right angles
to it. They are invariably flawed, so much so that
a flawless emerald has passed into proverb as un-
attainable perfection. The largest single crystal
which is known to exist at the present day is in the
possession of the Duke of Devonshire (Fig. 71). In
section it is nearly a regular hexagon, about 2 inches
(5 1 mm.) in diameter from side to side, and the
length is about the same; its weight is 27679
grams (9f oz. Av., or 1347 carats). It is of good
colour, but badly flawed. It was given to the Duke
of Devonshire by Dom Pedro of Brazil, and was
exhibited at the Great Exhibition of 1851. A
fine, though much smaller crystal, but of even better
192
GEM-STONES
colour, which weighs 32*2 grams (156^- carats),
and measures i-|. inch (28 mm.) in its widest cross-
diameter, and about the same in length, was acquired
with the Allan- Greg collection by the British
Museum, and is exhibited in the Mineral Gallery
FIG. 71. — Duke of Devonshire's Emerald.
(Natural size.)
of the British Museum (Natural History). The
finest cut emerald is said to be one weighing 30
carats, which belongs to the Czar of Russia. A
small, but perfect and flawless, faceted emerald,
which is set in a gold hoop, is also in the British
Museum (Natural History). It is shown, without
the setting, about actual size, on Plate I, Fig. 5.
BERYL 193
The ever great demand and the essentially re-
stricted supply have forced the cost of emeralds of
good quality to a height that puts large stones
beyond the reach of all but a privileged few who
have purses deep enough. The rate per carat may
be anything from £15 upwards, depending upon the
purity of the colour and the freedom from flaws, but
it increases very rapidly with the size, since flawless
stones of more than 4 carats or so in weight are
among the rarest of jewels ; a perfect emerald of 4
carats may easily fetch £1600 to ^2000. It seems
anomalous to say that it has never been easier to
procure fine stones than during recent years, but
the reason is that the high prices prevailing have
tempted owners of old jewellery to realize their
emeralds. On the other hand, pale emeralds are
worth only a nominal sum.
The other varieties of beryl are much less rare,
and, since they usually attain to more considerable,
and sometimes even colossal, size, far larger stones
are obtainable. An aquamarine, particularly of good
deep blue-green colour, is a stone of great beauty,
and it possesses the merit of preserving its purity
of tint in artificial light. It is a favourite stone for
pendants, brooches, and bracelets, and all purposes
for which a large blue or green stone is desired.
The varying tints are said to be due to the presence
of iron in different percentages, and possibly in
different states of oxidation. Unlike emerald, the
other varieties are by no means so easily recognized
by their colour. Blue aquamarines may easily be
mistaken for topaz, or vice versa, and the yellow
beryl closely resembles other yellow stones, such as
quartz, topaz, or tourmaline. Stones which are
13
194 GEM-STONES
colourless or only slightly tinted command little
more than the price of cutting, but the price of
blue-green stones rapidly advances with increasing
depth of tint up to £2 a carat: The enormous
cut aquamarine which is exhibited in the Mineral
Gallery of the British Museum (Natural History),
affords some idea of the great size such stones reach ;
a beautiful sea-green in colour, it weighs 179*5
grams (875 carats), and is table-cut with an oval
contour.
The splendid six-sided columns which have been
discovered in various parts of Siberia are among the
most striking specimens in any large mineral collec-
tion. The neighbourhood of Ekaterinburg in the
Urals is prolific in varieties of aquamarine ; especially
at Mursinka have fine stones been found, in associa-
tion with topaz, amethyst, and schorl, the black
tourmaline. Good stones also occur in conjunction
with topaz at Miask in the Government of Orenburg.
It is found in the gold-washings of the Sanarka
River, in the Southern Urals, but the stones are not
fitted for service as gems. Magnificent blue-green
and yellow aquamarines are associated with topaz
and smoky quartz in the granite of the Adun-
Tschilon Mountains, near Nertschinsk, Transbaikal.
Stones have also been found at the Urulga River in
Siberia. Most of the bluish-green aquamarines
which come into the market at the present time
have originated in Brazil, particularly in Minas
Novas, Minas Geraes, where clear, transparent stones,
of pleasing colour, in various shades, are found in
the utmost profusion ; beautiful yellow stones also
occur at the Bahia mines. Aquamarine was
obtained in very early times in Coimbatore District,
BERYL 195
Madras, India, and yellow beryl comes from Ceylon.
Fine blue crystals occur in the granite of the Mourne
Mountains, Ireland, but they are not clear enough
for cutting purposes ; similar stones are found also
at Limoges, Haute Vienne, France. Aquamarines
of various hues abound in several places in the
United States, among the principal localities being
Stoneham in Maine, Haddam in Connecticut, and
Pala and Mesa Grande in San Diego County,
California. The last-named state is remarkable for
the numerous stones of varying depth of salmon-
pink that have been found there. It is, however,
surpassed by Madagascar, which has recently pro-
duced splendid stones of perfect rose-red tint and
of the finest gem quality, some of them being
nearly 100 carats in weight. These stones, which
have been assigned a special name, morganite (cf.
supra), are associated with tourmaline and kunzite.
Pink and yellow beryls and deep blue-green aqua-
marines occur in the island in quantity. The pink
beryls from California are generally pale or have a
pronounced salmon tint, and seldom approach the
real rose-red colour of morganite ; one magnificent
rose-red crystal, weighing nearly 9 Ib. (4*05 kg.),
has, however, been recently discovered in San Diego
County, California, and is now in the British
Museum (Natural History). Blue-green beryl,
varying in tint from almost colourless to an
emerald-green, occurs with tin-stone and topaz
about 9 miles (14^ km.) north-east of Emmaville in
New South Wales, Australia.
Probably the largest and finest aquamarine crystal
ever seen was one found by a miner on March 28,
1910, at a depth of 15 ft. (5 m.) in a pegmatite vein
196 GEM-STONES
at Marambaya, near Arassuahy, on the Jequitinhonha
River, Minas Geraes, Brazil. It was greenish blue
in colour, and a slightly irregular hexagonal prism,
with a flat face at each end, in form ; it measured
19 in. (48'5 cm.) in length and 16 in. (41 cm.) in
diameter, and weighed 243 Ib. (iio-5 kg.); and its
transparency was so perfect that it could be seen
through from end to end (Plate XXVI). The
crystal was transported to Bahia, and sold for
$25,000 (£5133)-
PLATE XXVI
RGE AQUAMARINE CRYSTAL (one-sixth natural size), FOUND AT
MINAS GERAES, BRAZIL
PART II— SECTION B
SEMI-PRECIOUS STONES
CHAPTER XXI
TOPAZ
TOPAZ is the most popular yellow stone in
jewellery, and often forms the principal
stone in brooches or pendants, especially in old-
fashioned articles. It is a general idea that all
yellow stones are topazes, and all topazes are
yellow ; but neither statement is correct. A very
large number of yellow stones that masquerade as
topaz are really the yellow quartz known as citrine.
The latter is, indeed, almost universally called by
jewellers topaz, the qualification ' Brazilian ' being
used by them to distinguish the true topaz. Many
species besides those mentioned yield yellow stones.
Thus corundum includes the beautiful ' oriental
topaz' or yellow sapphire, and yellow tourmalines
are occasionally met with ; the yellow chrysoberyl
always has a greenish tinge. Topaz is generally
brilliant-cut in front and step-cut at the back, and
the table facet is sometimes rounded, but the
colourless stones are often cut as small brilliants;
it takes an excellent and dazzling polish.
198 GEM-STONES
Topaz is a silicate of aluminium corresponding
to the formula [Al(F,OH)]2SiO4, which was estab-
lished in 1894 by Penfield and Minor as the result
of careful research. Contrary to the general idea,
topaz is usually colourless or very pale in tint.
Yellow hues of different degrees, from pale to a
rich sherry tint (Plate I, Fig. 9), are common,
and pure pale blue (Plate I, Fig. 7) and pale green
stones, which often pass as aquamarine, are far
from rare. Natural, red and pink, stones are very
seldom to be met with. It is, however, a peculiarity
of the brownish-yellow stones from Brazil that the
colour is altered by heating to a lovely rose-pink.
Curiously, the tint is not apparent when the stone
is hot, but develops as it cools -to a normal tempera-
ture ; the colour seems to be permanent. Such
stones are common in modern jewellery. Although
the change in colour is accompanied by some slight
rearrangement of the constituent molecules, since
such stones are invariably characterized by high
refraction and pronounced dichroism, the crystalline
symmetry, however, remaining unaltered, the cause
must be attributed to some change in the tinctorial
agent, probably oxidation. The yellow stones from
Ceylon, if treated in a similar manner, lose their
colour entirely. The pale yellow-brown stones from
Russia fade on prolonged exposure to strong sun-
light, for which reason the superb suite of crystals
from the Urulga River, which came with the
Koksharov collection to the British Museum, are
kept under cover.
The name of the species is derived from topazion
(T07ra£en>, to seek), the name given to an island in
the Red Sea, which in olden times was with difficulty
TOPAZ
199
located, but it was applied by Pliny and his con-
temporaries to the yellowish peridot found there.
The term was applied in the Middle Ages loosely
to any yellow stone, and was gradually applied
more particularly to the stone that was then more
prevalent, the topaz of modern science. As has
already been pointed out (p. ill), the term is still
employed in jewellery to signify any yellow stone.
The true topaz was probably included by Pliny
under the name chrysolithus.
The symmetry is orthorhombic, and the crystals
are prismatic in shape and ter-
minated by numerous inclined
faces, and usually by a large face
perpendicular to the prism edge
(Fig. 72). Topaz cleaves with
great readiness at right angles
to the prism edge ; owing to its
facile cleavage, flaws are easily
started, and caution must be
exercised not to damage a stone
by knocking it against hard and unyielding sub-
stances. The dichroism of a yellow topaz is
always perceptible, one of the twin colours being
distinctly more reddish than the other, and the
phenomenon is very marked in the case of stones
the colour of which has been artificially altered to
pink. The values of the least and the greatest of
the principal indices of refraction vary from 1*615
to 1-629, and from 1-625 to 1-637, respectively,
the double refraction being about 0*010 in amount,
and positive in sign. The high values correspond
to the altered stones. The specific gravity, the
mean value of which is 3-55 with a variation of
\
FIG. 72.— Topaz
Crystal.
200 GEM-STONES
0*05 on either side, is higher than would be ex-
pected from the refractivity. A cleavage flake
exhibits in convergent polarized light a wide-
angled biaxial picture, the ' eyes ' lying outside
the field of view. The relation of the principal
optical directions and the directions of single re-
fraction to the crystal are shown in Fig. 27. The
hardness is 8 on Mohs's scale, and in this character
it is surpassed only by chrysoberyl, corundum,
and diamond. Topaz is pyro-electric, in which
respect tourmaline alone exceeds it, and it may be
strongly electrified by friction.
Although the range of refraction overlaps that
of tourmaline, there is no risk of confusion, because
the latter has nearly thrice the amount of double
refraction (cf. p. 29). Apart from the difference
in refraction, a yellow topaz ought never to be
confused with a yellow quartz, because the former
sinks, and the latter floats in methylene iodide.
The same test distinguishes topaz from beryl, and,
indeed, from tourmaline also.
Judged by the criterion of price, topaz is not in
the first rank of precious stones. Stones of good
colour and free from flaws are now, however, scarce.
Pale stones are worth very little, possibly less than
43. a carat, but the price rapidly advances with
increase in colour, reaching 2os. for yellow, 8os.
for pink and blue stones. Since topazes are pro-
curable in all sizes customary in jewellery, the rates
vary but slightly, if at all, with the size.
Topaz occurs principally in pegmatite dykes and
in cavities in granite, and is interesting to petrolo-
gists as a conspicuous instance of the result of the
action of hot acid vapours upon rocks rich in
TOPAZ 201
aluminium silicates. Magnificent crystals have
come from the extensive mining district which
stretches along the eastern flank of the Ural
Mountains, and from the important mining region
surrounding Nertschinsk, in the Government of
Transbaikal, Siberia. Fine green and blue stones
have been found at Alabashka, near Ekaterinburg,
in the Government of Perm, and at Miask in the
Ilmen Mountains, in the Government of Orenburg.
Topazes of the rare reddish hue have been picked
out from the gold washings of the Sanarka River,
Troisk, also in the Government of Orenburg.
Splendid pale-brown stones have issued from the
Urulga River, near Nertschinsk, and good crystals
have come from the Adun-Tschilon Mountains.
Kamchatka has produced yellow, blue, and green
stones. In the British Isles, beautiful sky-blue,
waterworn crystals have been found at Cairngorm,
Banffshire, in Scotland, and colourless stones in
the Mourne Mountains, Ireland, and at St. Michael's
Mount, Cornwall. Most of the topazes used in
jewellery of the present day come from either
Brazil or Ceylon. Ouro Preto, Villa Rica, and
Minas Novas, in the State of Minas Geraes, are
the principal localities in Brazil. Numerous stones,
often waterworn, brilliant and colourless or tinted
lovely shades of blue and wine-yellow, occur there ;
reddish stones also have been found at Ouro Preto.
Ceylon furnishes a profusion of yellow, light-green,
and colourless, waterworn pebbles. The colourless
stones found there are incorrectly termed by the
natives ' water-sapphire,' and the light-green stones
are sold with beryl as aquamarines ; the stones
locally known as ' king topaz ' are really yellow
202 GEM-STONES
corundum (cf. p. 181). Colourless crystals, some-
times with a faint tinge of colour, have been dis-
covered in many parts of the world, such as
Ramona, San Diego County, California, and Pike's
Peak, Colorado, in the United States, San Luis
Potosi in Mexico, and Omi and Otami-yama in
Japan.
CHAPTER XXII
SPINEL
(Balas-Ruby, Rubicelle)
SPINEL labours under the serious disadvantage
of being overshadowed at almost all points
by its opulent and more famous cousins, sapphire
and ruby, and is not so well known as it deserves
to be. The only variety which is valued as a
gem is the rose-tinted stone called balas-ruby (Plate
XXVII, Fig. 3), which is very similar to the true ruby
in appearance; they are probably often confused,
especially since they are found in intimate associa-
tion in nature. Spinels of other colours are not
very attractive to the eye, and are not likely to be
in much demand. Blue spinel (Plate XXVII, Fig. 4)
is far from common, but the shade is inclined to
steely-blue, and is much inferior to the superb tint
of the true sapphire. Spinel is very hard and
eminently suitable for a ring-stone, but is seldom
large and transparent enough for larger articles
of jewellery.
Spinel is an aluminate of magnesium corre-
sponding to the formula MgAl2O4, and therefore is
closely akin to corundum, alumina, and chrysoberyl,
aluminate of beryllium. The composition may,
however, vary considerably owing to the isomor-
303
204 GEM-STONES
phous replacement of one element by another ; in
particular, ferrous oxide or manganese oxide often
takes the place of some magnesia, and ferric oxide
or chromic oxide is found instead of part of the
alumina. When pure, spinel is devoid of colour,
but such stones are exceedingly rare. No doubt
chromic oxide is responsible for the rose-red hue
of balas-ruby, and also, when tempered by ferric
oxide, for the orange tint of rubicelle, and man-
ganese is probably the cause of the peculiar violet
colour of almandine-spinel. It is scarcely possible
to define all the shades between blue and red that
may be assumed by spinel. Stones which are rich
in iron are known as pleonaste or ceylonite ; they
are quite opaque, but are sometimes used for orna-
mental wear.
The name of the species comes from a diminutive
form of O-TTIVOS, a spark, and refers to the fiery red
colour of the most valued kind of spinel. It may
be noted that the Latin equivalent of the word,
carbunculus, has been applied to the crimson garnet
when cut en cabochon. Balas is derived from
Balascia, the old name for Badakshan, the district
from which the finest stones were brought in
mediaeval times.
Spinel, like diamond, belongs to the cubic system
of crystalline symmetry, and occurs in beautiful octa-
hedra, or in flat triangular-shaped plates (Figs. 73, 74)
the girdles of which are cleft at each corner, these
plates being really twinned octahedra. The refrac-
tion is, of course, single, and there is therefore no
double refraction or dichroism ; this test furnishes
the simplest way of discriminating between the
balas and the true ruby. Owing to isomorphous
SPINEL 205
replacement the value of the refractive index may
lie anywhere between 1716 and 1736. The lower
values, about 1720, correspond to the most trans-
parent red and blue stones ; the deep violet stones
have values above 1730. Spinel possesses little
colour-dispersion, or ' fire.' In the same way the
values of the specific gravity, even of the trans-
parent stones, vary between 3-5 and 37, but the
opaque ceylonite has values as high as 4*1. Spinel
is slightly softer than sapphire and ruby, and has
the symbol 8 on Mohs's scale, and it is scarcely
inferior in lustre
to these stones.
Spinel is easily
separated from
garnet of similar
colour by its
lower refractivity.
Spinels run from FlGS. 73j 74._spinel Crystals,
i os. to £5 a carat,
depending on their colour and quality, and excep-
tional stones command a higher rate.
Spinel always occurs in close association with
corundum. The balas and the true ruby are mixed
together in the limestones of Burma and Siam.
Curiously enough, the spinel despite its lower hard-
ness is found in the river gravels in perfect crystals,
whereas the rubies are generally waterworn. Fine
violet and blue spinels occur in the prolific gem-
gravels of Ceylon. A large waterworn octahedron
and a rough mass, both of a fine red colour, are
exhibited in the Mineral Gallery of the British
Museum (Natural History), and a beautiful faceted
blue stone is shown close by.
206 GEM-STONES
The enormous red stone, oval in shape, which is
set in front of the English crown, is not a ruby, as
it was formerly believed to be, but a spinel. It was
given to the gallant Black Prince by Pedro the
Cruel after the battle of Najera in 1367, and was
subsequently worn by Henry V upon his helmet at
the battle of Agincourt. As usual with Indian-
fashioned stones it is pierced through the middle,
but the hole is now hidden by a small stone of
similar colour.
The British Regalia also contains the famous stone
called the Timur Ruby or Khiraj-i-Alam (Tribute of
the World), which weighs just over 352 carats, and
is the largest spinel-ruby known. It is uncut, but
polished. Its history goes back to 1398, when it
was captured by the Amir Timur at Delhi. On the
wane of the Tartar empire the stone became the pro-
perty of the Shahs of Persia, until it was given by
Abbas I to his friend and ally, the Mogul Emperor,
Jehangir. It remained at Delhi until, on the sack of
that city by Nadir Shah in 1739, it, together with
immense booty, including the Koh-i-nor, fell into the
hands of the conqueror. Like the great diamond, it
eventually came into the possession of Runjit Singh
at Lahore, and on the annexation of the Punjab in
1850 passed to the East India Company. It was
shown at the Great Exhibition of 1851, and after-
wards presented to Queen Victoria.
Mention has been made above (p. 121) of the
blue spinel which is manufactured in imitation of
the true sapphire. The artificial stone is quite
different in tint from the blue spinel found in
nature.
CHAPTER XXIII
GARNET
THE important group of minerals which are
known under the general name of garnet
provides an apt illustration of the fact that rarity
is an essential condition if a stone is to be accounted
precious. Owing to the large quantity of Bohemian
garnets, of a not very attractive shade of yellowish
red, that have been literally poured upon the market
during the past half-century the species has become
associated with cheap and often ineffective jewellery,
and has acquired a stigma which completely pre-
vents its attaining any popularity with those pro-
fessing a nice taste in gem-stones. It must not,
however, be supposed that garnet has entirely dis-
appeared from high-class jewellery although the
name may not readily be found in a jeweller's
catalogue. Those whose business it is to sell gem-
stones are fully alive to the importance of a name,
and, as has already been remarked (p. 109), they
have been fain to meet the prejudices of their
customers by offering garnets under such misleading
guises as ' Cape-ruby,' ' Uralian emerald,' or ' olivine.'
Garnets may, moreover, figure under another
name quite unintentionally. Probably many a
fine stone masquerades as a true ruby ; the im-
possibility of distinguishing these two species in
208 GEM-STONES
certain cases by eye alone is perhaps not widely
recognized. An instructive instance came under
the writer's notice a few years ago. A lady one
day had the misfortune to fracture one of the stones
in a ruby ring that had been in the possession of
her family for upwards of a century, and was origin-
ally purchased of a leading firm of jewellers in
London. She took the ring to her jeweller, and
asked him to have the stone replaced by another
ruby. A day or two later he sent word that it
was scarcely worth while to put a ruby in because
the stones in the ring were paste. Naturally dis-
tressed at such an opinion of a ring which had
always been held in great esteem by her family,
the lady consulted a friend, who suggested showing
it to the writer. A glance was sufficient to prove
that if the ring had been in use so long the stones
could not possibly be paste on account of the
excellent state of their polish, but a test with the
refractometer showed that the stones were really
almandine-garnets, which so often closely resemble
the true ruby in appearance. Beautiful as the
stones were, the ring was probably not worth one-
tenth what the value would have been had the
stones been rubies.
To the student of mineralogy garnet is for many
reasons of peculiar interest. It affords an excellent
illustration of the facility which certain elements
possess for replacing one another without any great
disturbance of the crystalline form. Despite their
apparent complexity in composition all garnets con-
form to the same type of formula : lime, magnesia,
and ferrous and manganese oxides, and again alumina
and ferric and chromic oxides may replace each other
GARNET 209
in any proportion, iron being present in two states
of oxidation, and it would be rare to find a stone
which agrees in composition exactly with any of
the different varieties of garnet given below.
Garnet belongs to the cubic system of crystalline
symmetry. Its crystals are commonly of two kinds,
both of which are very characteristic, the regular
dodecahedron, i.e. twelve-faced figure (Fig. 75), and
the tetrakis-octahedron or three-faced octahedron
(Fig. 76); the latter crystals are, especially when
weather- or water- worn, almost spherical in shape.
Closer and more refined observations have shown
that garnet is sel-
dom homogeneous,
being usually com-
posed of several
distinct individuals
of a lower order
of symmetry. Al-
though singly re-
fractive as far as. can be determined. with the refracto-
meter or by deviation through a prism, yet when
examined under the polarizing microscope, garnets
display invariably a small amount of local double
refraction. The transition from light to darkness is,
however, not sharp as in normal cases, but is pro-
longed into a kind of twilight. In hardness, garnet
is on the whole about the same as quartz, but varies
slightly ; hessonite and andradite are a little softer,
pyrope, spessartite, and almandine are a little harder,
while uvarovite is almost the same. All the varieties
except uvarovite are fusible when heated before
the blowpipe, and small fragments melt sufficiently
on the surface in the ordinary bunsen flame to
210 GEM-STONES
adhere to the platinum wire holding them. This
test is very useful for separating rough red garnets,
pyrope or almandine, from red spinels or zircons
of very similar appearance. Far greater variation
occurs in the other physical characters. The specific
gravity may have any value between 3*55 and
4-20, and the refractive index ranges between 1740
and I '890. Both the specific gravity and the re-
fractive index increase on the whole with the per-
centage amount of iron.
Garnet is a prominent constitutent of many
kinds of rocks, but the material most suitable for
gem purposes occurs chiefly in crystalline schists or
metamorphic limestones. Pyrope and demantoid are
furnished by peridotites and the serpentines result-
ing from them ; almandine and spessartite come
mostly from granites.
The name of the species is derived from the
Latin granatus, seed-like, and is suggested by the
appearance of the spherical crystals when embedded
in their pudding-like matrix.
The varieties most adapted to jewellery are the
fiery-red pyrope and the crimson and columbine-red
almandine ; the closer they approach the ruddy hue
of ruby the better they are appreciated. Hessonite
was at one time in some demand, but it inclines too
much to the yellowish shade of red and possesses
too little perfection of transparency to accord with
the taste of the present day. Demantoid provides
beautiful, pale and dark emerald-green stones, of
brilliant lustre and high dispersion, which are
admirably adapted for use in pendants or necklaces ;
on account of their comparative softness it would be
unwise to risk them in rings. In many stones the
GARNET 211
colour takes a yellowish shade, which is less in
demand. Uvarovite also occurs in attractive
emerald-green stones, but unfortunately none as yet
have been found large enough for cutting. A few
truly magnificent spessartites are known — one, a
splendid example, weighing 6f- carats, being in the
possession of Sir Arthur Church ; but the species
is far too seldom transparent to come into general
use. The price varies per carat from 2s. for
common garnet to IDS. for stones most akin to
ruby in colour, and exceptional demantoids may
realize even as much as £10 a carat. The old style
of cutting was almost invariably rounded or en
cabochon, but at the present day the brilliant-cut front
and the step-cut back is most commonly adopted.
The several varieties will now be considered in
detail.
(a) HESSONITE
(Grossular, Cinnamon- Stone, Hyacinth, JacintJt)
This variety, strictly a calcium-aluminium garnet
corresponding to the formula Ca3Al2(SiO4)3, but
generally containing some ferric oxide and there-
fore tending towards andradite, is called by several
different names. In science it is usually termed
grossular, a word derived from grossularia, the
botanical name for gooseberry, in allusion to
the colour and appearance of many crystals, or
hessonite, and less correctly essonite, words derived
from the Greek r\a<rwv in reference to the inferior
hardness of these stones as compared with zircon
of similar colour ; in jewellery it is better known
as cinnamon-stone, if a golden-yellow in colour, or
hyacinth or jacinth. The last word, which is in-
212 GEM-STONES
discriminately used for hessonite and yellow zircon,
but should more properly be applied to the latter,
is derived from an old Indian word (cf. p. 229);
jewellers, however, retain it for the garnet.
Only the yellow and orange shades of hessonite
(Plate XXIX, Fig. 5) are used for jewellery. Neither
the brownish-green kind, to which the term grossular
may properly be applied, nor the rose-red is trans-
parent enough to serve as a gem-stone. Hessonite
may mostly be recognized, even when cut, by the
curiously granular nature of its structure, just as if
it were composed of tiny grains imperfectly fused
together ; this appearance, which is very character-
istic, may readily be perceived if the interior of the
stone be viewed through a lens of moderate power.
The specific gravity varies from 3-55 to 3-66,
and the refractive index from 1742 to 1*748. The
hardness is on the whole slightly below that of
quartz. When heated before a blowpipe it easily
fuses to a greenish glass.
The most suitable material is found in some
profusion in the gem-gravels of Ceylon, in which it
is mixed up with zircon of an almost identical
appearance; both are called hyacinth. Hessonites
from other localities, although attractive as museum
specimens, are not large and clear enough for cutting
purposes. Switzerland at one time supplied good
stones, but the supply has long been exhausted.
(£) PYROPE
(' Cape-Ruby ')
Often quite ruby - red in colour (Plate XXIX,
Fig. 6), this variety is probably the most popular of
GARNET 213
the garnets. It is strictly a magnesium-aluminium
garnet corresponding to the formula Mg3Al2(SiO4)3,
but usually contains some ferrous oxide and thus
approaches almandine. Both are included among
the precious garnets. Its name is derived from
TTvprn-jrof, fire-like, in obvious allusion to its
characteristic colour.
Although at its best pyrope closely resembles
ruby, its appearance is often marred by a tinge of
yellow which decidedly detracts from its value.
Pyrope generally passes as a variety of ruby, and
under such names as ' Cape-ruby,' ' Arizona-ruby,'
depending on the origin of the stones, commands a
brisk sale. The specific gravity varies upwards
from 3*70, depending upon the percentage amount
of iron present, and similarly the refractive index
varies upwards from 1740; in the higher values
pyrope merges into almandine. Its hardness is
slightly greater than that of quartz, and may be
expressed on Mohs's scale by the symbol 7^.
An enormous quantity of small red stones,
mostly with a slight tinge of yellow, have been
brought to light at Teplitz, Aussig, and other spots
in the Bohemian Mittelgebirge, and a considerable
industry in cutting and marting them has grown
up at Bilin. Fine ruby-red stones accompany
diamond in the ' blue ground ' of the mines at
Kimberley and also at the Premier mine in the
Transvaal. Similar stones are also found in
Arizona and Colorado in the United States, and in
Australia, Rhodesia, and elsewhere.
Although commonly quite small in size, pyrope
has occasionally attained to considerable size. Ac-
cording to De Boodt the Kaiser Rudolph II had one
214 GEM-STONES
in his possession valued at 45,000 thalers (about
£6750). The Imperial Treasury at Vienna con-
tains a stone as large as a hen's egg. Another
about the size of a pigeon's egg is in the famous
Green Vaults at Dresden, and the King of Saxony
has one, weighing 46 8 £ carats, set in an Order of
the Golden Fleece.
(c) RHODOLITE
This charming pale-violet variety was found at
Cowee Creek and at Mason's Branch, Macon County,
North Carolina, U.S.A., but in too limited amount
to assume the position in jewellery it might other-
wise have expected. In composition it lies between
pyrope and almandine, and may be supposed to
contain a proportion of two molecules of the
former to one of the latter. Its specific gravity is
3-84, refractive index 1760, and hardness 7\. It
exhibits in the spectroscope the absorption-bands
characteristic of almandine.
(d) ALMANDINE
(Carbuncle)
This variety is iron-aluminium garnet correspond-
ing to the formula Fe3Al2(SiO4)3, but the com-
position is very variable. In colour it is deep
crimson and violet or columbine-red (Plate XXIX,
Fig. 8), but with increasing percentage amount of
ferric oxide it becomes brown and black, and opaque,
and quite unsuitable for jewellery. The name of
the variety is a corruption of Alabanda in Asia
Minor, where in Pliny's time the best red stones
GARNET 2 1 5
were cut. Almandine is sometimes known as
Syriam, or incorrectly Syrian garnet, because at
Syriam, once the capital of the ancient kingdom of
Pegu, which now forms part of Lower Burma,
such stones were cut and sold. Crimson stones,
cut in the familiar en cabochon form and known as
carbuncles, were extensively employed for enrich-
ing metalwork, and a half-century or so ago were
very popular for ornamental wear, but their day has
long since gone. Such glowing stones are aptly
described by their name, which is derived from the
Latin carbunculus, a little spark. In Pliny's time,
however, the term was used indiscriminately for all
red stones. It has already been remarked that the
word spinel has a similar significance.
The specific gravity varies from 3-90 for trans-
parent stones to 4*20 for the densest black stones,
and the refractive index may be as high as r8io.
Almandine is one of the hardest of the garnets, and
is represented by the symbol 7| on Mohs's scale.
The most interesting and curious feature of
almandine lies in the remarkable and characteristic
absorption-spectrum revealed when the transmitted
light is examined with a spectroscope (p. 61).
The phenomenon is displayed most vividly by the
violet stones, and is, indeed, the cause of their
peculiar colour.
Although a common mineral, almandine of a
quality fitted for jewellery occurs in comparatively
few localities. It is found in Ceylon, but not so
plentifully as hessonite. Good stones are mined in
various parts of India, and are nearly all cut at
Delhi or Jaipur. Brazil supplies good material,
especially in the Minas Novas district of Minas
216 GEM-STONES
Geraes, where it accompanies topaz, and Uruguay
also furnishes serviceable stones. Almandine is
found in Australia, and in many parts of the
United States. Recently small stones of good
colour have been discovered at Luisenfelde in
German East Africa.
(e) SPESSARTITE
Properly a manganese-aluminium garnet corre-
sponding to the formula Mn3Al2(SiO4)3, this
variety generally contains iron in both states of
oxidation. If only transparent and large enough
its aurora-red colour would render it most accept-
able in jewellery. Two splendid stones have, in-
deed, been found in Ceylon (p. 21 1), and good stones
rather resembling hessonites have been quarried at
Amelia <£ourt House in Virginia, and others have
come from Nevada ; otherwise, spessartite is un-
known as a gem-stone.
The specific gravity ranges from 4*0 to 4*3, and
the refractive index is about r8i, both characters
being high ; the hardness is slightly greater than
that of quartz.
(/) ANDRADITE
(Demantoid, Topazolile, ' Olivine ')
Andradite is strictly a calcium-iron garnet corre-
sponding to the formula Ca3Fe2(SiO4)3, but as
usual the composition varies considerably. It is
named after d'Andrada, a Portuguese mineralogist,
who made a study of garnet more than a century
ago.
GARNET 217
Once contemptuously styled common garnet, and-
radite suddenly sprang into the rank of precious
stones upon the discovery some thirty years ago of
the brilliant, green stones (Plate XXIX, Fig. 7) in
the serpentinous rock beside the Bobrovka stream, a
tributary of the Tschussowaja River, in the Sissersk
district on the western side of the Ural Mountains.
The shade of green varies from olive through
pistachio to a pale emerald, and is probably due to
chromic oxide. Its brilliant lustre, almost challeng-
ing that of diamond, and its enormous colour-
dispersion, in which respect it actually transcends
diamond, raise it to a unique position among
coloured stones. Unfortunately its comparative
softness limits it to such articles of jewellery as
pendants and necklaces, where it is not likely to be
rubbed. When first found it was supposed to be
true emerald, which does actually occur near
Ekaterinburg, and was termed ' Uralian emerald.'
When analysis revealed its true nature, it received
from science the slightly inharmonious name of
demantoid in compliment to its adamantine lustre.
Jewellers, however, prefer to designate it ' olivine,'
not very happily, because the stones usually cut are
not olive-green and the name is already in extensive
use in science for a totally distinct species (p. 225);
they recognized the hopelessness of endeavouring to
find a market for them as garnets. The yellow
kind of andradite known as topazolite would be an
excellent gem-stone if only it were found large and
transparent enough. Ordinary andradite is brown
or black, and opaque ; it has occasionally been used
for mourning jewellery.
The specific gravity varies from 3'8 to 3-9, being
2 1 8 GEM-STONES
about 3*85 for demantoid, which has a high refractive
index, varying from i'88o to 1-890, and may with
advantage be cut in the brilliant form. It is the
softest of the garnets, being only 6^ on Mohs's
scale.
(g) UVAROVITE
This variety, which is altogether unknown in
jewellery, is a calcium-iron garnet correspond-
ing mainly to the formula Ca3Cr2(SiO4)3, but with
some alumina always present, and was named
after a Russian minister. It has an attractive green
colour, and is, moreover, hard, being about /| on
Mohs's scale, but it has never yet come to light
of a size suitable for cutting. The specific gravity
is low, varying from 3^4 1 to 3' 5 2. Unlike the
kindred varieties it cannot be fused by heating
before an ordinary blowpipe.
CHAPTER XXIV
TOURMALINE
(Rubellite)
nr^OURMALINE is unsurpassed even by co-
X rundum in variety of hue, and it has during
recent years rapidly advanced in public favour,
mainly owing to the prodigal profusion in which
nature has formed it in that favoured State,
California, the garden of the west. Its comparative
softness militates against its use in rings, but its
gorgeous coloration renders it admirably fitted for
service in any article of jewellery, such as a brooch
or a pendant, in which a large central stone is
required. Like all coloured stones it is generally
brilliant-cut in front and step-cut at the back, but
occasionally it is sufficiently fibrous in structure
to display, when cut en cabochon, pronounced
chatoyancy.
The composition of this complex species has
long been a vexed question among mineralogists, but
considerable light was recently thrown on the sub-
ject by Schaller, who showed that all varieties of
tourmaline may be referred to a formula of the
type 1 2Si02.3B203.(9 -^)[(Al,Fe)203].34(Fe,Mn)Ca,
Mg,K2,Na2)Li2,H2)O].3H2O. The ratios of boric
oxide, silica, and water are nearly constant in all
220 GEM-STONES
analyses, but great variation is possible in the
proportions of the other constituents. Having
regard to this complexity, it is not surprising to
find that the range in colour is so great Colourless
stones, to which the name achroite is sometimes
given, were at one time exceedingly rare, but they
are now found in greater number in California.
Stones which are most suited to jewellery purposes
are comparatively free from iron, and apparently
owe their wonderful tints to the alkaline earths ;
lithia, for instance, is responsible for the beautiful
tint of the highly prized rubellite, and magnesia, no
doubt, for the colour of the brown stones of various
tints. Tourmaline rich in iron is black and almost
opaque. It is a striking peculiarity of the species
that the crystals are rarely uniform in colour
throughout, the boundaries between the differently
coloured portions being sharp and abrupt, and the
tints remarkably in contrast. Sometimes the
sections are separated by planes at right angles to
the length of the crystal, and sometimes they are
zonal, bounded by cylindrical surfaces running
parallel to the same length. In the latter case a
section perpendicular to the length shows zones of
at least three contrasting tints. In the Brazilian
stones the core is generally red, bounded by white,
with green on the exterior, while the reverse is the
case in the Californian stones, the core being green
or yellow, bounded by white, with red on the
exterior. Tourmaline may, indeed, be found of
almost every imaginable tint, except, perhaps, the
emerald green and the royal sapphire-blue. The
principal varieties are rose-red and pink (rubellite)
(Plate XXVII, Fig. i), green (Brazilian emerald),
TOURMALINE 221
indigo-blue (indicolite), blue (Brazilian sapphire),
yellowish green (Brazilian peridot) (Plate XXVII,
Fig. 2), honey-yellow (Ceylonese peridot), violet-red
(siberite), and brown (Plate XXVII, Fig. 8). The
black, opaque stones are termed schorl.
The name of the species is derived from the
Ceylonese word, turamali, and was first employed
when a parcel of gem-stones was brought to
Amsterdam from Ceylon in 1703 ; in Ceylon,
however, the term is applied by native jewellers to
the yellow zircon commonly found in the island.
Schorl, the derivation of which is unknown, is the
ancient name for the species, and is still used in
that sense by miners, but it has been restricted by
science to the black variety. The 'Brazilian
emerald ' was introduced into Europe in the
seventeenth century and was not favourably received,
possibly because the stones were too dark in colour
and were not properly cut ; that they should have
been confused with the true emerald is eloquent
testimony to the extreme ignorance of the characters
of gem-stones prevalent in those dark ages.
Achroite comes from the Greek, a%/3oo?, without
colour.
To the crystallographer tourmaline is one of the
most interesting of minerals. If the crystals, which
are usually prismatic in form, are doubly terminated,
the development is so obviously different at the two
ends (Fig. 77) as to indicate that directional character
in the molecular arrangement, termed the polarity,
which is borne out by other physical properties.
Tourmaline is remarkably dichroic. A brown
stone, except in very thin sections, is practically
opaque to the ordinary ray, and consequently a
222
GEM-STONES
J
section cut parallel to the crystallographic axis, i.e.
to the length of a crystal prismatically developed,
transmits only the extraordinary ray. Such sections
were in use for yielding plane-polarized light before
Nicol devised the calcite prism known by his name
(cf. p. 44). It is evident that tourmaline, unless very
light in tint, must be cut with the table facet
parallel to that axis, because otherwise the stone
will appear dark and lifeless. The values of the
extraordinary and ordinary refrac-
tive indices range between i'6i4
and i -63 8, and 1-633 and 1*669
respectively ; the double refraction,
therefore, is fairly large, amounting
to O'O25, and, since the ordinary
exceeds the extraordinary ray, its
character is negative. The specific
gravity varies from 3'O to 3 '2. The
lower values in both characters
correspond to the lighter coloured
stones used in jewellery ; the black
stones, as might be expected from
their relative richness in iron, are the
densest. The hardness is only about the same as that
of quartz, or perhaps a little greater, varying from 7 to
7-|. It will be noticed that the range of refractivity
overlaps that of topaz (q.v.\ but the latter has a
much smaller double refraction, and may thus be
distinguished (p. 29). Unmounted stones are still
more easily distinguished, because tourmaline floats
in methylene iodide, while topaz sinks. The pyro-
electric phenomenon (cf. p. 82) for which tourmaline
is remarkable, although of little value as a test in
the case of a cut stone, is of great scientific interest,
FIG. 77.— Tourma-
line Crystal.
TOURMALINE 223
because it is strong evidence of the peculiar
crystalline symmetry pertaining to its molecular
arrangement. Tourmalines range in price from 53.
to 2Os. a carat according to their colour and quality,
but exceptional stones may command a higher rate.
Tourmaline is usually found in the pegmatite
dykes of granites, but it also occurs in schists and
in crystalline limestones. Rubellite is generally
associated with the lithia mica, lepidolite ; the
groups of delicate pink rubellite bespangling a
background of greyish white lepidolite are among
the most beautiful of museum specimens. Mag-
nificent crystals of pink, blue, and green tourmaline
have been found in the neighbourhood of Ekaterin-
burg, principally at Mursinka, in the Urals, Russia,
and fine rubellite has come from the Urulga River,
and other spots near Nertschinsk, Transbaikal,
Asiatic Russia. Elba produces pink, yellowish,
and green stones, frequently particoloured ; some-
times the crystals are blackened at the top, and
are then known locally as 'nigger-heads.' Ceylon
supplies small yellow stones — -the original tourmaline
— which are confused with the zircon of a similar
colour, and rubellite accompanies the ruby at Ava,
Burma. Beautiful crystals, green and red, often
diversely coloured, come from various parts, such as
Minas Novas and Arassuhy, of the State of Minas
Geraes, Brazil. Suitable gem material has been
found in numerous parts of the United States.
Paris and Hebron in Maine have produced gorgeous
pink and green crystals, and Auburn in the same
state has supplied deep-blue, green, and lilac stones.
Fine crystals, mostly green, but also pink and
particoloured, occur in an albite quarry near the
224 GEM-STONES
Conn River at Haddam Neck, Connecticut. All
former localities have, however, been surpassed by
the extraordinary abundance of superb green,
and especially pink, crystals at Pala and Mesa
Grande in San Diego County, California. As
elsewhere, many-hued stones are common. The
latter locality supplies the more perfectly trans-
parent crystals. Kunz states that two remarkable
rubellite crystals were found there, one being 45
mm. in length and 42 mm. in diameter, and the
other 56 mm. in length and 24 mm. in diameter.
Madagascar, which has proved of recent years to
be rich in gem-stones, supplies green, yellow, and
red stones, both uniformly tinted and particoloured,
which in beauty, though perhaps not in size, beat
comparison with any found elsewhere.
CHAPTER XXV
PERIDOT
THE beautiful bottle-green stone, which from its
delicate tint has earned from appreciative
admirers the poetical sobriquet of the evening
emerald, and which has during recent years crept
into popular favour and now graces much of the
more artistic jewellery, is named as a gem-stone
peridot — a word long in use among French jewellers,
the origin and meaning of which has been forgotten
— but is known to science either as olivine, on
account of the olive-green colour sometimes
characterizing it, or as chrysolite. It is of interest
to note that the last word, derived from xpvvos,
golden, and Xt'0o9, stone, was in use at the time of
Pliny, but was employed for topaz and other yellow
stones, while his topaz, curiously enough, designated
the modern peridot (cf. p. 1 99), an inversion that has
occurred in other words. The true olivine must not
be confused with the jewellers' 'olivine,' which is
a green garnet from the Ural Mountains (p. 2 1 7).
Peridot is comparatively soft, the hardness varying
from 6 1 to 7 on Mohs's scale, and is suitable only
for articles which are not likely to be scratched ; the
polish of a peridot worn in a ring would soon
deteriorate. The choicest stones are in colour a
lovely bottle-green (Plate XXIX, Fig. 2) of various
15 "5
226 GEM-STONES
depths; the olive-green stones (Plate XXIX, Fig. 3)
cannot compare with their sisters in attractiveness.
The step form of cutting is considered the best for
peridot, but it is sometimes cut round or oval in
shape, with brilliant-cut fronts.
Peridot is a silicate of magnesium and iron,
corresponding to the formula (Mg,Fe)2SiO4, ferrous
iron, therefore, replacing magnesia. To the ferrous
iron it is indebted for its colour, the pure magnesium
silicate being almost colourless, and the olive tint
arises from the oxidation of the iron. The latitude
in the composition resulting from this replacement
is evinced in the considerable range that has been
observed in the physical characters, but the crystal-
line symmetry persists unaltered ; the lower values
correspond to the stones that are usually met with
as gems. Peridot belongs to the orthorhombic
system of crystalline symmetry, and the crystals,
which display a large number of faces, are prismatic
in form and generally somewhat flattened. The
stones, however, that come into the market for
cutting as gems are rarely unbroken. The dichroism
is rather faint, one of the twin colours being slightly
more yellowish than the other, but it is more pro-
nounced in the olive-tinted stones. The values of
the least and greatest of the principal indices of re-
fraction vary greatly, from 1*650 and r683 to
1-668 and 1701, but the double refraction, amount-
ing to 0-033, remains unaffected. Peridot, though
surpassed by sphene in extent of double refraction,
easily excels all the ordinary gem-stones in this
respect, and this character is readily recognizable in
a cut stone by the apparent doubling of the opposite
edges when viewed through the table facet (cf.
PLATE X.\\'U
t. KUBEI.LJTE 2- TOURMALINS
8. TOURMALINE
5. BALAS-RUBY
4. BLUE SPINEL
7 AMETHYST
10. KIRK-OPAI
ALEXANDRITE
(Ky daylight)
13- ALEXANDRITE
fRy artificial HsM,
CHKVSOBERYL
:EM-STO\E3
PERIDOT 227
p. 41). An equally large variation occurs in the
specific gravity, namely, from 3-3 to 3-5.
Peridots of deep bottle-green hue command
moderate prices at the present day, about 303. a
carat being asked for large stones ; the paler tinted
stones run down to a few shillings a carat. The
rate per carat may be very much larger for stones
of exceptional size and quality.
Olivine, to use the ordinary mineralogical term, is
a common and important constituent of certain
kinds of igneous rocks, and it is also found in those
strange bodies, meteorites, which come to us from
outer cosmical space. Except in basaltic lavas, it
occurs in grains and rarely in well-shaped crystals.
Stones that are large and transparent enough for
cutting purposes come almost entirely from the
island Zebirget or St. John situated on the west
coast of the Red Sea, opposite to the port of
Berenice. This island belongs to the Khedive of
Egypt, and is at present leased to a French
syndicate. It is believed to be the same as the
mysterious island which produced the 'topaz' of
Pliny's time. Magnificent stones have been dis-
covered here, rich green in colour, and 20 to 30,
and occasionally as much as 80, carats in weight
when cut; a rough m~ss attained to the large
weight of 190 carats. Pretty, light-green stones
are supplied by Queensland, and peridots of a less
pleasing dark-yellowish shade of green, and without
any sign of crystal form, have during recent years
come from North America. Stones rather similar
to those from Queensland have latterly been found
in the Bernardino Valley in Upper Burma, not far
from the ruby mines.
CHAPTER XXVI
ZIRCON
{Jargoon, Hyacinth, JacintJi)
ZIRCON, which, if known at all in jewellery, is
called by its variety names, jargoon and
hyacinth or jacinth, is a species that deserves greater
recognition than it receives. The colourless stones
rival even diamond in splendour of brilliance and
display of ' fire ' ; the leaf-green stones (Plate XXIX,
Fig- J3) possess a restful beauty that commends
itself; the deep-red stones (Plate XXIX, Fig. 14), if
somewhat sombre, have a certain grandeur ; and no
other species produces such magnificent stones of
golden-yellow hue (Plate XXIX, Fig. 12). Zircon is
well known in Ceylon, which supplies the world with
t<£he finest specimens, and is highly appreciated by the
Tmabitants of that sunny isle, but it scarcely finds
a place in jewellery elsewhere. The colourless
stones are cut as brilliants, but brilliant-cut fronts
with step-cut backs is the usual style adopted for the
coloured stones.
Zircon is a silicate of zirconium corresponding to
the formula ZrSiO4, but uranium and the rare earths
are generally present in small quantities. The aurora-
red variety is known as hyacinth or jacinth, and the
term jargoon is applied to the other transparent
ZIRCON 229
varieties, and especially to the yellow stones. The
most attractive colours shown by zircon are leaf-
green, golden-yellow, and deep red. Other common
colours are brown, greenish, and sky-blue. Colour-
less stones are not found in nature, but result from
the application of heat to the yellow and brown
stones.
The name of the species is ancient, and comes
from the Arabic zarqun, vermilion, or the Persian
zargun, gold-coloured. From the same source in all
probability is derived the word jargoon through the
French jargon and the Italian giacone. Hyacinth
(cf. p. 2 1 1 ) is transliterated from the Greek vdxivOo?,
itself adapted from an old Indian word; it is in no
way connected with the flower of the same name.
The last word has seen some changes of meaning.
In Pliny's time yellow zircons were indiscriminately
classified with other yellow stones as chrysolite.
His hyacinth was used for the sapphire of the
present day, but was subsequently applied to any
transparent corundum. Upon the introduction of
the terms, sapphire and ruby, for the blue and the
red corundum hyacinth became restricted to the
other varieties, of which the yellow was the,
commonest. In the darkness of the Middle Agis
it was loosely employed for all yellow stones
emanating from India, and was finally, with increas-
ing discernment in the characters of gem-stones,
assigned to the yellow zircon, since it was the
commonest yellow stone from India.
Considered from the scientific point of view, zircon
is by far the most interesting and the most remark-
able of the gem-stones. The problem presented by
its characters and constitution is one that still awaits
230 GEM-STONES
a satisfactory solution. Certain zircons, which are
found as rolled pebbles in Ceylon and never show
any trace of crystalline faces, have very nearly single
refraction, and the values of the refractive index
vary from 1*790 to i'84O, and the specific gravity
is about 4-00 to 4- 14, and the hardness is slightly
greater than that of quartz, being about 7^-. On
the other hand, such stones as the red zircons from
Expailly have remarkably different properties.
They show crystalline faces with tetragonal
symmetry, the faces present being four prismatic
faces mutually intersecting at right angles and four
inclined faces at each end (Fig. 78).
They have large double refraction,
varying from 0*044 to 0*062, which is
readily discerned in a cut stone (cf.
p. 41), and the refractive indices are
high, the ordinary index varying from
1*923 to 1*931 and the extraordinary
from 1*967 to 1*993. Since the
ordinary is less than the extraordin-
ary index the sign of the double refraction is
positive. The specific gravity likewise is much
higher, varying from 4*67 to 4*71. The second
type, therefore, sinks in molten silver- thallium
nitrate, whereas the first type floats. The second
type is also slightly harder, being about 7^ on
Mohs's scale. By heating either of these types the
physical characters are not much altered, except that
the colour is weakened or entirely driven off and
some change takes place in the double refraction.
But between these two types may be found zircons
upon which the effect of heating is striking. They
seem to contract in size so that the specific gravity
ZIRCON 231
increases as much as three units in the first place of
decimals, and a corresponding increase takes place
in the refractive indices, and in the amount of double
refraction. The cause of these changes remains a
matter of speculation. Evidently a third type of
zircon exists which is capable of most intimate
association with either of the other types, and which
is very susceptible to the effect of heat. It may be
noted that stones of the intermediate type are
usually characterized by a banded or zonal structure
suggesting a want of homogeneity. The theory has
been advanced that zircon contains an unknown
element which has not yet been separated from
zirconium. Zircon of the first type favours green,
sky-blue, and golden-yellow colours; honey-yellow,
light green, blue, and red colours characterize the
second type ; and the intermediate stones are mostly
yellowish green, cloudy blue, and green.
It is another peculiarity of zircon that it some-
times shows in the spectroscope absorption bands
(p. 61), which were observed in 1866 by Church.
Many zircons do not exhibit the bands at all, and
others only display the two prominent bands in the
red end of the spectrum.
Of all the gem-stones zircon alone approaches
diamond in brilliance of lustre, and it also possesses
considerable ' fire ' ; it can, of course, be readily
distinguished by its inferior hardness, but a judg-
ment based merely on inspection by eye might
easily be erroneous.
According to Church, who has made a lifelong
study of zircon, the green and yellowish stones of
the first variety emit a brilliant orange light when
being ground on a copper wheel charged with
232 GEM-STONES
diamond dust, and the golden stones of the inter-
mediate type glow with a fine orange incandescence
in the flame of a bunsen burner ; the latter pheno-
menon is supposed to be due to the presence of
thoria.
The leaf-green stones almost invariably show a
series of parallel bands in the interior.
Zircons vary from 53. to 155. a carat, but
exceptional stones may be worth more.
By far the finest stones come from Ceylon.
The colourless stones are there known as ' Matura
diamonds,' and the hyacinth includes garnet
(hessonite) of similar colour, which is found with it
in the same gravels. The stones are always water-
worn. Small hyacinths and deep-red stones come
from Expailly, Auvergne, France, and yellowish-red
crystals are found in the Ilmen Mountains, Oren-
burg, Russia. Remarkably fine red stones have
been discovered at Mudgee, New South Wales, and
yellowish-brown stones accompany diamond at the
Kimberley mines, South Africa.
CHAPTER XXVII
CHRYSOBERYL
(Chrysolite, Cats -Eye, Cymophane, Alexandrite)
CHRYSOBERYL has at times enjoyed fleeting
popularity on account of the excellent cat's-
eyes cut from the fibrous stones, and in the form of
alexandrite it meets with a steadier, if still limited,
demand. It is a gem-stone that is seldom met with
in ordinary jewellery, although its considerable
hardness befits it for all such purposes.
Chrysoberyl is in composition an aluminate of
beryllium corresponding to the formula BeAl2O4,
and is therefore closely akin to spinel. It usually
contains some ferric and chromic oxides in place of
alumina, and ferrous oxide in place of beryllia, and
it is to these accessory constituents that its tints are
due. Other gem-stones containing the uncommon
element beryllium are phenakite and beryl. Pale
yellowish green, the commonest colour, is supposed
to be caused by ferrous oxide ; such stones are known
to jewellers as chrysolite (Plate XXVII, Fig. 12).
Cat's-eyes (Plate XXIX, Fig. i) have often also a
brownish shade of green. The bluish green and dark
olive-green stones known as alexandrite (Plate XXVII,
Figs. II, 13) differ in appearance so markedly from
their fairer sisters that their common parentage seems
234 GEM-STONES
almost incredible. The dull fires that glow within
them, and the curious change that comes over them
at night, add a touch of mystery to these dark
stones. Chromic oxide is held responsible for their
colour. The cat's-eyes are, of course, always cut en
cabochon, but otherwise chrysoberyl is faceted.
The name of the species is composed of two
Greek words, xpvcros, golden, and /3ijpv\\o<;, beryl,
and etymologically more correctly defines the lighter-
coloured stones, which were, indeed, at one time
the only kind known. Chrysolite from ^puo-o?,
golden, and \/0o9, stone, has much the same signifi-
cance. This name is preferred by jewellers, but in
science it is applied to an entirely different species,
which is known in jewellery as peridot. Cymo-
phane, from Kvpa, wave, and <f>aiveiv, appear, refers
to the peculiar opalescence characteristic of cat's-
eyes ; it is sometimes used to designate these stones,
but does not find a place within the vocabulary of
jewellery. Alexandrite is named after Alexander
II, Czar of Russia, because it first came to light on
his birthday. That circumstance, coupled with its
display of the national colours, green and red, and
its at one time restriction to the mining district near
Ekaterinburg, renders it dear to the heart of all
loyal Russians.
Chrysoberyl crystallizes in the orthorhombic
system, and occurs in rather dull, complex crystals,
which are sometimes so remarkably twinned, especi-
ally in the variety called alexandrite, as to simulate
hexagonal crystals. In keeping with the crystalline
symmetry it is doubly refractive and biaxial, having
two directions of single refraction. The least and
the greatest of the principal indices of refraction
CHRYSOBERYL 235
may have any values between 1742 and 1749,
and 1750 and 1757, respectively, the maximum
amount of double refraction remaining always the
same, namely, 0*009. The mean principal refractive
index is close to the least ; the sign of the double
refraction is therefore positive, and the shadow-edge
corresponding to the lower index, as seen in the
refractometer, has little, if any, perceptible motion
when the stone is rotated. The converse is the
case with corundum ; the sign is negative, and it is
the shadow-edge corresponding to the greater re-
fractive index that remains unaltered in position on
rotation of the stone. This test would suffice to
separate chrysoberyl from yellow corundum, even if
the refractive indices of the former were not sensibly
lower than those of the latter. Also, the dichroism
of chrysolite is stronger than that of yellow
sapphires. In alexandrite this phenomenon is most
prominent; the absorptive tints, columbine-red,
orange, and emerald-green, corresponding to the
three principal optical directions, are in striking con-
trast, and the first differs so much from the intrinsic
colour of the stone as to be obvious to the unaided
eye, and is the cause of the red tints visible in a cut
stone. The curious change in colour of alexandrite,
from leaf-green to raspberry-red, that takes place
when the stone is seen by artificial light, is due to
a different cause, as has been pointed out above
(p. 54). The effect is illustrated by Figs. 1 1, 13 on
Plate XXVII, which represent a fine Ceylon stone as
seen by daylight and artificial light; the influence
of dichroism may be noticed in the former picture.
The specific gravity of chrysoberyl varies from 3'68
to 378. In hardness this species ranks above spinel
236 GEM-STONES
and comes next to corundum, being given the
symbol 8J on Mohs's scale. Certain stones contain
a multitude of microscopic channels arranged in
parallel position. When the stones are cut with
their rounded surface parallel to the channels, a
broadish band of light is visible running across the
stone at right angles to them, and suggests the pupil
of a cat's eye, whence the common name for the
stones. The fact that the channels are hollow
causes an opalescence, which is absent from the
quartz cat's-eye.
The most important locality for the yellowish
chrysoberyl is the rich district of Minas Novas,
Minas Geraes, Brazil, where it occurs in the form of
pebbles, and excellent material is also supplied by
Ceylon, in both crystals and rounded pebbles.
Other places for chrysolite are Haddam, Connecti-
cut, and Greenfield, Saratoga County, New York,
in the United States, and recently in the gem-
gravels near the Somabula Forest, Rhodesia.
Ceylon supplies some of the best cat's-eyes. Alex-
andrite was first discovered, as already stated, at the
emerald mines near Ekaterinburg, in the Urals ; but
the supply is now nearly exhausted. A poorer
quality comes from Takowaja, also in the Urals.
Good alexandrite has come to light in Ceylon, and
most of the stones that are placed on the market at
the present day have emanated from that island.
The Ceylon stones reach a considerable size, often
as much as from 10 to 20 carats in weight; the
Russian stones have a better colour and are more
beautiful, but they are less transparent, and rarely
exceed a carat in weight. Good chrysolite may be
worth from IDS. to £2 a carat, and cat's-eye runs
CHRYSOBERYL 237
from £1 to £4 a carat, depending upon the quality.
Alexandrites meet with a steady demand in Russia,
and fine stones are scarce ; flawless stones about a
carat in weight are worth as much as £30 a carat,
and even quite ordinary stones fetch £4. a carat.
From Ceylon, that interesting home of gems, have
originated some magnificent chrysoberyls, including
a superb chrysolite, 8of carats in weight, and
another, a splendid brownish yellow in colour and
very even in tint, and two large alexandrites, green
in daylight and a rich red by night, weighing 63!
and 28|-f carats. The finest cut chrysolite existing
is probably the one exhibited in the Mineral Gallery
of the British Museum (Natural History). Abso-
lutely flawless and weighing 43! carats, it was
formerly contained in the famous Hope collection, and
is described on page 56 and figured on Plate XXI
of the catalogue prepared by B. Hertz, which was
published in 1839 ; the weight there given includes
the brilliants and the ring in which it was mounted.
It is shown, about actual size, in Plate XXVII, Fig. 1 2.
A magnificent cat's-eye, 3 5 '5 by 3 5 mm. in size, which
also formed part of the Hope collection, was included
in the crown jewels taken from the King of Kandy
in 1815. The crystalline markings in the cut stone
are so arranged that the lower half shows an altar
overhung by a torch. The stone has been famous
in Ceylon for many ages. It was set in gold with
rubies cut en cabochon. Two fine Ceylon alexand-
rites of exceptional merit, weighing 42 and 26f
carats, are also exhibited in the Mineral Gallery of
the British Museum (Natural History). The former
is illustrated in Plate XXVII, Figs. 1 1, 1 3, as seen in
daylight and in artificial light.
CHAPTER XXVIII
QUARTZ
(Rock-Crystal, Amethyst, Citrine, Cairngorm, Cafs-
Eye, Tigers-Eye)
A LTHOUGH the commonest and, in its natural
£\. form, the most easily recognizable of mineral
substances, quartz nevertheless holds a not incon-
spicuous position among gem-stones, because, as
amethyst (Plate XXVII, Fig. 7), it provides stones of
the finest violet colour ; moreover, the yellow quartz
(Plate XXVII, Fig. 5) so ably vies with the true topaz
that it is universally known to jewellers by the name
of the latter species, and is too often confounded
with it, and the lustrous, limpid rock-crystal even
aspires to the local title of ' diamond.' For all
purposes where a violet or yellow stone is required,
quartz is admirably suited ; it is hard and durable,
and it has the merit, or possibly to some minds the
drawback, of being moderate in price. Despite its
comparative lack of ' fire,' rock-crystal might replace
paste in rings and buckles with considerable advan-
tage from the point of view of durability. The
chatoyant quartz, especially in the form known as
tiger's-eye, will for beauty bear comparison with the
true cat's-eye, which is a variety of chrysoberyl.
Except that cat's-eye is cut en cabochon, quartz is
step- or sometimes brilliant-cut.
238
QUARTZ 239
Ranking with corundum next to diamond as the
simplest in composition of the gem-stones, quartz is
the crystallized form of silica, oxide of silicon, corre-
sponding to the formula SiO2. When pure, it is
entirely devoid of the faintest trace of colour and
absolutely water- clear. Such stones are called rock-
crystal, and it is easy to understand why in early
days it was supposed to represent a form of petrified
water. It is these brilliant, transparent stones that
are, when small, known in many localities as
'diamonds.' Before the manufacture of glass was
discovered and brought to perfection, rock-crystal
was in considerable use for fashioning into cups,
vases, and so forth. The beautiful tints character-
izing quartz are due to the usual metallic oxides.
To manganese is given the credit of the superb
purple or violet colour of amethyst, which varies
considerably in depth. Jewellers are inclined to
distinguish the deep-coloured stones with the prefix
1 oriental,' but the practice is to be deprecated, since
it might lead to confusion with the true oriental
amethyst, which is a purple sapphire, one of the
rarest varieties of corundum. Quartz of a yellow
hue is properly called citrine, but, as already stated,
jewellers habitually prefer the name ' topaz ' for it,
and distinguish the true topaz by the prefix
Brazilian — not a very happy term, since both the
yellow topaz and the yellow quartz occur plentifully
in Brazil. Sometimes the yellow quartz is termed
occidental, Spanish, or false topaz. Stones with a
brownish or smoky tinge of yellow are called
cairngorm, or Scotch topaz. The colour of the
yellow stones is doubtless due to a trace of ferric
oxide. Stones of a smoky brown colour are known
240 GEM-STONES
as smoky-quartz. Rose-quartz, which is rose-red or
pink in colour and hazy in texture, is comparatively
rare ; strange to say, it has never been found in
distinct crystals. The tint, which may be due to
titanium, is fugitive, and fades on exposure to strong
sunlight. In milky quartz, as the name suggests,
the interior is so hazy as to impart to the stone a
milky appearance. It has frequently happened that
quartz has crystallized after the formation of other
minerals, with the result that the latter are found
inside it. Prase, or mother-of-emerald, which at one
time was supposed to be the mother-rock of emerald,
is a quartz coloured leek-green by actinolite fibres
in the interior. Specimens containing hair-like
fibres of rutile — the so-called fleches d'amour — are
common in mineral collections, and are sometimes
to be seen worked. When enclosing a massive,
light-coloured, fibrous mineral, the stones have a
chatoyant effect, and display, when suitably cut, a
fine cat's-eye effect ; in tiger's-eye the enclosed
mineral is crocidolite, an asbestos, the original blue
hue of which has been changed to a fine golden-
brown by oxidation. Quartz which contains scales
of mica, hematite, or other flaky mineral has a vivid
spangled appearance, and is known as aventurine ;
it has occasionally been employed for brooches or
similar articles of jewellery. Rainbow-quartz, or
iris, is a quartz which contains cracks, the chromatic
effect being the result of the interference of light
reflected from them ; it has been artificially produced
by heating the stone and suddenly cooling it.
The name of the species is an old German mining
term of unknown meaning which has been in general
use in all languages since the sixteenth century.
QUARTZ 241
Amethyst is derived from apeOva-ros, not drunken,
possibly from a foolish notion that the wearer was
exempt from the usual consequences of unrestrained
libations. Pliny suggests as an alternative explana-
tion that its colour approximates to, but does not
quite reach, that of wine. Aventurine, from aventura,
an accident, was first applied to glass spangled with
copper, the effect being said to have been acci-
dentally discovered owing to a number of copper
filings falling into a pot of molten glass in a Venetian
factory.
Quartz belongs to the hexagonal system of
crystalline symmetry, and crystal-
lizes in the familiar six-sided prisms
terminated by six inclined, often
triangular, faces (Fig. 79) ; twins are
common, though they are not always
obvious from the outward develop-
ment. In accordance with the sym- _
e L. . ill FlG- 79-— Quartz
metry the refraction is double, and Crystal.
there is one direction of single re-
fraction, namely, that parallel to the edge of the
prism. The ordinary refractive index has the
value i '544, and the extraordinary i'5S3, and since
the latter is the greater, the sign of the double
refraction is positive. The double refraction is
small in amount, but is large enough to enable
the apparent doubling of certain of the opposite
edges of a faceted stone to be perceptible when
viewed with a lens through the table-facet. The
dichroism of the deep-coloured stones is quite
distinct. Quartz has only about the same amount
of colour dispersion as ordinary glass, and lacks,
therefore, 'fire.' The application of strong heat
16
242 GEM-STONES
tends, as usual, to weaken or drive off the colour.
Thus the dense smoky-quartz found in Spain, Brazil,
and elsewhere is converted into stones of a colour
varying from light yellow to reddish brown accord-
ing to the amount and duration of the application.
In the case of amethyst the colour is changed to a
deep orange, or entirely driven off if the temperature
be high enough. Its density is very constant, vary-
ing only from 2*654 to 2'66o ; the purest stones
are the lightest. To it has been assigned the symbol
7 on Mohs's scale of hardness.
To physicists quartz is one of the most interesting
of minerals because of its power of rotating, to an
extent depending upon the thickness of the section,
the plane of polarization of a beam of light tra-
versing it in a direction parallel to the prism edge.
It appears, moreover, from a study of the pyro-
electric and general physical characters, that its
molecular structure has a helical arrangement, which,
like all screws, may have a right- or left-handed
character. Amethyst is, in fact, invariably composed
of separate twin individuals, alternately right- and
left-handed ; in some remarkable crystals the section
at right angles to the prism edge is composed of
triangular sectors, alternately of different hands
and of different tints — purple and white. To the
twinning is due the rippled fracture and the feathery
inclusions so characteristic of amethyst.
Besides its use for ornamental purposes, quartz
finds a place as the material for lenses intended for
delicate photographic work, because its transparency
to the ultra-violet light is so much greater than that
of glass. Spectacle lenses made of it are in demand,
because they are not liable to scratches, and retain,
QUARTZ 243
therefore, their polish indefinitely. When fused in the
oxyhydrogen flame, quartz becomes a silica glass, of
specific gravity 2 -2 and hardness 5 on Mohs's scale,
which has proved of great service for laboratory
ware, because it withstands sudden and unequal
heating without any danger of fracture ; it has also
in fine threads been invaluable for delicate torsion
work, because it acquires not the smallest amount of
permanent twist, in this respect being superior to
the finest silk threads.
Clear rock-crystal fetches little more than the
cost of the cutting ; citrine and amethyst are worth
from is. to 53. a carat, depending upon the quality
and size of the stone; smoky-quartz is practically
valueless; rose-quartz realizes less than is. a carat ;
and the value of cat's-eye is also small — only is.
to 2s. 6d. a carat. Tiger's-eye at one time com-
manded as much as 253. a carat, but the supply
exceeded the demand, with the consequent collapse
in the price.
Beautiful, brilliant, and limpid rock-crystal is
found in various parts of the world : in the Swiss
Alps, at Bourg d'Oisans in the Dauphine" Alps,
France, in the famous Carrara marble, in the Mar-
maros Comitat of Hungary, and in the United
States, Brazil, Madagascar, and Japan. Small
lustrous stones, known in their localities as ' Isle
of Wight,' ' Cornish,' or ' Bristol diamonds,' are found
in our own country. Brazil supplies stones out
of which have been cut the clear balls used
in crystal-gazing. The finest amethysts come
from Brazil — especially the State of Rio Grande
do Sul — and from Uruguay, India, and the gem-
gravels of Ceylon; good stones also occur at
244 GEM-STONES
Ekaterinburg, in the Ural Mountains. A splendid
Brazilian amethyst, weighing 334 carats, and two
Russian stones — one hexagonal in contour, weighing
88 carats, and the other, a deep purple in colour
with a circular table, weighing 73 carats — are
exhibited in the British Museum (Natural History).
Cairngorm is known from the place of that name
in Banffshire, Scotland, whence fine specimens have
emanated ; it is a gem much valued in that country.
Fine cairngorm has also originated from Pike's
Peak, Colorado. Splendid yellow stones have had
their birth in the States of Minas Geraes, Sao Paulo,
and Goyaz, of Brazil — especially in the last. The
fine Spanish smoky-quartz, which, as already stated,
turns yellow on heating, comes from Hinojosa, in
the Province of Cordova. The delicate rose-quartz
is known at Bodenmais in Bavaria, Paris in Maine,
United States, and Ekaterinburg in the Ural
Mountains. The finest cat's-eyes are found in India
and Ceylon, and are high in favour with the natives.
Greenish stones of an inferior quality are brought
from the Fichtelgebirge in Bavaria, and are sold as
' Hungarian cat's-eyes,' despite the fact that no
such stone occurs in Hungary — another instance of
jewellers' disdain for accuracy. Tiger's-eye occurs
in considerable quantity in the neighbourhood of
Griquatown, Griqualand West, South Africa. A
silicified ctocidolite, in which the blue colour is
retained, comes also from Salzburg, and is known as
sapphire- or azure-quartz, or siderite.
Certain of the pebbles found on the seashore of
our coasts, especially off the Isle of Wight and
North Wales, cut into attractive, clear stones, more
or less yellow in colour ; but examples suitable for
QUARTZ 245
the purpose are not so numerous as might be
supposed, and do not reward any casual search.
Les affaires sont les affaires. The local lapidary,
instead of explaining that the pebbles brought to
him are not worth cutting, finds it more convenient
and profitable to substitute for them other, inferior
and badly .cut, stones, bought by the gross, or even
paste stones ; the customer, on the other hand, is
contented with a pretty bauble, and is not grateful
for the information that it might have been obtained
for a fraction of the sum paid.
CHAPTER XXIX
CHALCEDONY, AGATE, ETC.
/CHALCEDONY and agate, and their endless
V ' varieties, are composed mainly of silica, but
the separate individual crystals are so small as
to be invisible to the unaided eyesight, and occasion-
ally are so extremely minute that the structure is
almost amorphous. The colour and appearance vary
greatly, depending upon the impurities contained in
the stone, and, since both have been made a criterion
for differentiation of types, a host of names have
come into use, none of which are susceptible of
strict definition. On the whole, these stones may
be divided into two groups : chalcedony, in which
the structure is concretionary and the colour
comparatively uniform, and agate, in which the
arrangement takes the form of bands, varying
greatly in tint and colour.
The refraction, though double in the individual,
is irregular over the stone as a whole, and the indices
approximate to 1*550. The specific gravity ranges
from 2*62 to 2'64, depending upon the impurities
present. The degree of hardness is about the same as
that of quartz, namely, 7 on Mohs's scale. All kinds
are more or less porous, and stones of a dull colour
are therefore artificially tinted after being worked.
The term chalcedony, derived from ^a\K^atv
246
CHALCEDONY, AGATE, ETC. 247
the name of a town in Asia Minor, is usually
confined to stones of a greyish tinge. Stones
artifically coloured an emerald green have been cut
and put upon the market as ' emeraldine.' Carnelian
is a clear red chalcedony, and sard is somewhat
similar, but brownish in tint. Chrysoprase is apple-
green in colour, nickel oxide being supposed to be
the agent. Prase (cf. p. 240), which is a dull leek-
green in hue, may also in part be referred here ;
the name comes from irpdcrpov, a leek. Plasma,
which may have the same derivation, is a brighter
leek-green. Jasper is a chalcedony coloured blood-
red by iron oxide, while bloodstone is a green
chalcedony spotted with jasper; they are popular
stones for signet rings. Flint, an opaque
chalcedony, breaks with a sharp cutting edge, and
was much in request with early man as a tool or a
weapon ; its property of giving sparks when struck
with steel rendered it invaluable before the invention
of matches. Hornstone is somewhat similar, but
more brittle, while chert is a flinty rock.
Agate, named after the river Achates in Sicily,
where it was found at the time of Theophrastus,
has a peculiar banded structure, the bands being
usually irregular in shape, following the configura-
tion of the cavity in which it was formed. Moss-
agate, or mocha-stone, contains moss-like inclusions
of some fibrous mineral. Onyx is an agate with
regular bands, the layers having sharply different
colours ; when black and white, it has, in days gone
by, been employed for cameos. Sardonyx is similar
in structure, but red and white in colour. Agate is
used in delicate balances for supporting the steel
knife-edges of the balance itself and of the pan-
248 GEM-STONES
holders, and is largely employed — especially when
artificially coloured — for umbrella handles and similar
articles.
Chalcedony and agate are found the whole world
over, but India, and particularly Brazil, are noted
for their fine carnelians and agates.
CHAPTER XXX
OPAL
(White Opal, Black Opal, Fire-Opal)
THAT opal in early times excited keen
admiration is evident from Pliny's enthusi-
astic description of these stones : " For in them
you shall see the burning fire of the carbuncle, the
glorious purple of the amethyst, the green sea of
the emerald, all glittering together in an incredible
mixture of light." During much of last century,
owing to the foolish superstition that ill-luck dogs
the footsteps of the wearer, the species lay under a
cloud, which has even now not quite dispersed, but
exercises a prejudicial effect upon the fortunes of
the stone. It has, however, recently attracted
considerable attention owing to the discovery of the
splendid black opals in Australia ; at one moment
black with the darkness of night, at the next by a
chance movement glowing with vivid crimson
flame, such stones may justly be considered the
most remarkable in modern jewellery. At the
present day opal is divided by jewellers roughly
into two main groups : ' white ' (Plate XXVII, Fig. 6)
and ' black' (Plate XXVII, Fig. 9), according as the
tint is light or dark, fire-opal (Plate XXVII, Fig. 10)
standing in a separate category.
250 GEM-STONES
Opal differs from the rest of the principal gem-
stones in being not a crystalline body, but a
solidified jelly, and it depends for its attractiveness
upon the characteristic play of colour, known, in
consequence, as opalescence (cf. p. 39), which
arises from a peculiarity in the structure. Opal is
mainly silica, SiO2, in composition, but contains in
addition an amount of water varying in precious
opal from 6 to 10 per cent. As the original jelly
cooled, it became riddled throughout with cracks,
which were afterwards generally filled with opal
matter, containing a different amount of water, and
therefore differing slightly in refractivity from the
original substance. The structure not being quite
homogeneous, each crack has the same action upon
light as a soap-film, and gives rise to precisely
similar phenomena ; the thinner and more uniform
the cracks, the greater the splendour of the chromatic
display, the particular tint depending upon the
direction in which the stone is viewed. The cracks
in certain opals were not filled up, and therefore
contain air. Such stones appear opaque and devoid
of opalescence until plunged into water ; they are
consequently known as hydrophane, from vScop, water,
and <j>aive<r0ai, to make appear. Owing to the effect
of total -reflection, light was stopped on the hither
side of the cracks before they were filled with water,
which is not far inferior to opal in refractivity; it
is surprising how much water these stones will
absorb.
Opal is colourless when pure, but is nearly
always more or less milky and opaque, or tinted
various dull shades by ferric oxide, magnesia or,
alumina. The so-called black opal is generally a
OPAL 251
dark grey or blue, and very rarely quite black.
That the coloration is not due to ordinary absorp-
tion, but to the action of cracks in the stone, is
shown by the fact that the transmitted light is
complementary to the reflected light ; the blue
opal is, for instance, a yellow when held up so that
light has passed through it. In many black opals
the opalescent material occurs in far too tiny pieces
to be cut separately, and the whole iron-stained
matrix is cut and polished and sold under the name
' opal-matrix.' The reddish and orange-coloured
stones known as fire-opal have pronounced colour
and only slight milkiness ; they display the
customary opalescence in certain directions. These
stones are often faceted, but otherwise opals are
cut en cabochon, either flat or steep — generally the
former in brooches and pendants, and the latter in
rings. Opal is somewhat soft, varying from 5 to 6£
on Mohs's scale, and is therefore easily scratched.
The specific gravity ranges from 2*10 to 2-20, and
the refractive index from 1*444 to 1*464, the
refraction, of course, being always single. It is
unwise to immerse opals in liquids on account of
their porosity.
The name opal comes to us through the Latin
opallus, which was used for the same species as
understood by the term at the present day, but the
word has a far older origin, which has not been
traced. The Romans also called the mineral
pczderos, the Greek form of Cupid, a name applied
to all rosy stones. The name cacholong, for the
bluish-white procelain variety, which is very porous
and adheres to the tongue, is of Tartar origin ; the
stone is highly valued in the East.
252 GEM-STONES
The oldest mines, which up to quite a recent
date were the only extensive deposit of opal known,
were at Cserwenitsa, near Kashau, in Hungary.
From them in all probability emanated the opals
known to the Romans. The opals from this locality
were generally quite small, and large pieces were
rare and commanded high prices. The Hungary
mines, however, proved quite unable to compete
with the rich fields at White Cliffs, New South
Wales, in spite of the efforts that were made to
depreciate and exclude from the market the new
stones, and at the present time few of the opals on
the market come from them. As so often happens,
the White Cliffs deposit was discovered by accident.
In 1889 a hunter, when tracking a wounded
kangaroo, chanced to pick up an attractively coloured
opal. The district is so waterless and forbidding
that, but for such a chance, the opals might have
long lain hidden. They occur in seams in deposits
of Cretaceous Age in a variety of ways, filling
cavities in rocks or sandstones, or cracks in wood,
or replacing wood, saurian bones, and some spiky
mineral, which may have been glauberite. In
recent years, another rich deposit was discovered
farther north, on both sides of the boundary between
Queensland and New South Wales. The field is-
remarkable for the darkness of its opals, which are
called ' black opal ' in contradistinction to the
lighter-coloured stones previously known. From
Lightning Ridge in New South Wales come stones
stained deep black which quite merit the designa-
tion black opal. The sandstone in which they
are found is rich in iron, and this is no doubt re-
sponsible for the deepness of their tint. Mexico is
PL A TE XX VII I
OPAL 253
noted for the fire-opal, which is found at Esperanza,
Queretaro, and Zimapan ; but other kinds of opal
also are found at these places.
The price of opal varies greatly, according to
the intrinsic colour and the uniformity and brilliance
of the opalescence. Common opal can be bought
at as low a rate as is. a carat, while black opal
ranges from I os. to £8 a carat ; but a good dark
stone displaying a flaming opalescence commands
a fancy figure, fine stones of this class being ex-
ceedingly rare. Fire-opal enjoys only a limited
popularity now, though a few years ago it was in
some demand; the price runs from 2s. to los. a
carat.
CHAPTER XXXI
FELSPAR
(Moonstone, Sunstone, Labradorite, Amazon- Stone)
THOUGH second to none among minerals in
scientific interest, whether regarded from
the point of view of their crystalline characters or
the important part they play in the formation of
rocks, the group included under the general name
felspar occupies but a humble place in jewellery. It
consists of three distinct species, orthoclase, albite,
and anorthite, which are silicates of aluminium, and
potassium, sodium, or calcium, corresponding to
the formulae KAlSi3O8, NaAlSi3O8, and CaAl2Si2O8
respectively, and also of species intermediate in
composition between albite and orthoclase, or albite
and anorthite. While differing in crystalline
symmetry, all are characterized by two directions
of cleavage which are nearly at right angles to one
another. The double refraction, which is slight in
amount, is biaxial in character and variable in sign.
The values of the least and greatest of the indices
of refraction range between 1*52 and i'53, and 1*53
and I '5 5 respectively, the double refraction at the
same time varying from 0*007 to 0*012. The
specific gravity lies between 2*48 and 2*66, and
FELSPAR 255
the hardness ranges between the degrees 6 and 7
on Mohs's scale.
Moonstone (Plate XXIX, Fig. 4), which is mainly
pure orthoclase, alone is at all common in jewellery.
It forms such an admirable contrasting frame for
large coloured stones that it deserves greater
popularity ; no doubt the cheapness of the stones
militates against their proper appreciation. The
milky, bluish opalescence from which they take
their name is caused by the reflection of light at
the thin twin-lamellae of which the structure is
composed. They are always cut more or less
steeply en cabochon. The finest stones were at one
time cut from the felspar that came from the St.
Gothard district in Switzerland and was in con-
sequence known as adularia from the neighbouring
Adular Mountains, somewhat incorrectly, since none
occurs at the latter locality. At the present day
practically all the moonstones on the market come
from Ceylon. They run in price from £3 to £20
per oz. (28 grams).
Sunstone is a felspar containing flakes of hematite
or goethite which impart a spangled bronze appear-
ance to the stones. Good material occurs in parts
of Norway. The remarkable sheen of labradorite
or blue felspar has its origin in the interference of
light at lamellar surfaces in the interior ; the uni-
formity of the colour over comparatively large areas
testifies to the regularity of the lamellar arrange-
ment. The finest specimens were brought from
the Isle of St. Paul off the coast of Labrador, where
they were first discovered in 1770; large masses
also occur on the coast itself. Amazon-stone is an
opaque green felspar which occurs in the Ilmen
256 GEM-STONES
Mountains, Orenburg, Russia, and at Pike's Peak,
Colorado, United States. It obtains its name from
the Amazon River, where, however, none has ever
been found ; there may have been some confusion
with a jade or similar stone.
Occasionally clear colourless felspar has been
faceted, and then closely resembles rock-crystal.
A careful determination of the refractive indices
and the specific gravity serves to discriminate
between them
PLATE .\.\IX
4. MOONSTONE
S. HESSONJTE 5. I'YROl'E
[I. IIIDDENITE
14. ZIRCON 15. ANDAI.US1TE
10. NEPHRIT!
(1KM-STONICS
CHAPTER XXXII
TURQUOISE, ODONTOLITE, VARISC1TE
OF all the opaque stones turquoise (Plate XXIX,
Fig. 17) alone finds a prominent place in
jewellery and can aspire to rank with the precious
stones. The colour varies from a sky-blue or a
greenish blue to a yellowish green or apple-green.
Only the former tints, which are at the same time
the rarer, are in general demand, and they possess
the great advantage of harmonizing with the tint
of the gold setting. The blue colours are, especially
in the case of the Siberian stones, by no means
permanent, and fade in course of time. Turquoise
is amorphous and seldom crystalline, and is therefore
somewhat porous ; it should consequently never
be immersed in liquids or be contaminated with
greasy and dirty matter lest the dreaded change
of colour be brought about. The stones are trans-
lucent in thin sections, and a good observation is
possible with the refractometer if the back of the
stone is flat and polished, since only the section
immediately adjacent to the instrument is concerned ;
the refractive index is about r6.l. The specific
gravity varies from 275 to 2-89. Turquoise has
a hardness of slightly under 6 on Mohs's scale,
and takes a good polish, which is fairly durable,
since on account of the comparative opacity of the
17 2S7
258 GEM-STONES
stones scratches on the surface are not very notice-
able. In composition it is a complex phosphate of
aluminium and copper, corresponding to the formula
CuOH.[6Al(OH)2].H5.(PO4)4, with ferric oxide replac-
ing some alumina The blue colour is due to the
copper constituent, and the predominance of iron
may cause the greenish shades ; but the water
contained in the stones plays no mean part, since
they turn a dirty green when it is driven off
The faded colour can sometimes be restored by
immersion of the stone in ammonia and subse-
quent application of grease, but the effect is not
lasting. Attempts are sometimes made to improve
inferior stones by impregnating them with Berlin
blue, but with only qualified success. Turquoises
are said to be affected by the perspiration from
the skin.
The name of the species comes from a French
word meaning Turkish, and arises from the fact that
the gem-stone first reached Europe by way of Turkey.
Another, but less obvious, suggestion is that it is
derived from the Persian name for the species,
piruzeh. Our turquoise and other phosphates of
similar appearance were probably known to Pliny
under the three names callais, callaina> and callaica.
The finest turquoise still comes from the famous
mines near Nishapur in the Persian province of
Khorassan, where it was known in very ancient
times; it is found with limonite filling the cracks
and cavities in a brecciated porphyritic trachyte.
Pieces of the turquoise and limonite from here are
sometimes cut without removal of the latter, and
sold as ' turquoise-matrix,' when the precious stones
are too tiny to be worth separate working. It also
TURQUOISE, ODONTOLITE, VARISCITE 259
occurs at Serbal in the Sinai Peninsula. Among
the more recent localities may be mentioned Los
Cerillos Mountains, New Mexico; Sierra Nevada,
Nevada, where pale blue and green stones are
found ; San Bernardino County, California, where
again the stones are rather pale ; and Arizona,
where it occurs in pale greenish-blue stones.
Some of the stones that have been seen are not
the true turquoise but odontolite, or bone turquoise,
which consists of the teeth and bones of mastodon
or other extinct animals, phosphate of iron being
the colouring material. These stones may easily
be recognized by their organic structure, which is
clearly visible if viewed with a strong lens or under
the microscope. Moreover, odontolite invariably
contains some calcium carbonate, and effervescence
takes place if it be touched with hydrochloric acid.
Turquoise dissolves in hydrochloric acid, but
without effervescence, and since it contains copper,
a fine blue colour is imparted to the solution by
the addition of ammonia. Odontolite has a higher
specific gravity, 3*0 to 3*5, but lower hardness,
5 on Mohs's scale.
Variscite, the hydrated phosphate of aluminium,
corresponding to the formula A1PO4+ 2H2O, is found
in masses resembling a greenish turquoise, but it
is much softer, being only 4 on Mohs's scale. The
specific gravity is 2'55. Round nodular masses of
variscite are found in Utah.
CHAPTER XXXIII
JADE
THOUGH not usually accounted precious
among European nations or in Western
civilization in general, jade was held in extraordi-
nary esteem by primitive man, and was fashioned
by him into ornaments and utensils, often of con-
siderable beauty, and even at the present day it
ranks among the Chinese and Japanese peoples
above all precious stones ; indeed, the Chinese word
Yu and the Japanese words Giyuku or Tama
signify both jade and precious stones in general.
According to the Chinese, jade is the prototype of all
jems, and unites in itself the five cardinal virtues —
Jin, charity ; Gi, modesty ; Yu, courage ; Ketsu,
justice ; and Chi, wisdom. When powdered and
mixed with water, it is supposed to be a powerful
remedy for all kinds of internal disorders, to
strengthen the frame and prevent fatigue, to prolong
life, and, if taken in sufficient quantity just before
death, to prevent decomposition.
Jade is a general term that includes properly two
distinct mineral species, nephrite or greenstone,
and jadeite, which are very similar in appearance,
both being fibrous and tough in texture, and more
or less greenish in colour ; but it is also applied
to other species such as saussurite, californite,
JADE 261
bowenite, and plasma, which have somewhat similar
characters. The word jade is a corruption of the
Spanish pietra di hijada, kidney-stone, in allusion
to its supposed efficacy in diseases of that organ.
Nephrite or greenstone (Plate XXIX, Fig. 16)
is the commoner of the two jades. It is closely
allied to the mineral hornblende, a silicate of
magnesium, iron, and calcium corresponding to the
formula Ca(Mg,Fe)3(SiO3)4, the magnesia being re-
placeable by ferrous oxide. Microscopic examina-
tion shows that the structure consists of innumerable
independent fibres foliated or matted together, the
former character giving rise to a slaty and the
latter to a horny appearance in the stone as seen
by the unaided eye. The colour varies from grey
to leaf- and dark-green, the tint deepening as the
relative amount of iron in the composition increases,
and brown tints result from the oxidation of the
iron along cracks in the stone. The hardness is
6 \ on Mohs's scale; nephrite is therefore about as
hard as ordinary glass and softer than quartz.
When polished, it always acquires a greasy lustre.
The specific gravity ranges from 2-9 to 3-1. The
least and greatest of the principal refractive indices
are I '606 and i'632 respectively, the double
refraction being biaxial and negative ; the coloured
fibres also display dichroism. All these differential
effects are, however, masked in the stone because of
the irregularity of the aggregation. Nephrite is
fusible before the blowpipe, but only with difficulty.
Its name is derived from the Greek word ve<f>po<i,
kidney, the allusion being the same as for jade.
Many of the prehistoric implements found in
Mexico and in the Swiss Lake Habitations are
262 GEM-STONES
composed of nephrite, but it is uncertain where the
mineral was obtained. Much of the material used
by the Chinese at the present time comes from
spots near the southern boundary of Eastern
Turkestan, especially in the valleys of the rivers
Karakash and Yarkand in the Kwen Lun range of
mountains; it is also found farther north at the
river Kashgar. It occurs in various provinces
of China, namely, Shensi, Kwei Chau, Kwang Tung,
Yunnan, and Manchuria. Gigantic waterworn
boulders have been found in the Government of
Irkutsk, near Lake Baikal, in eastern Siberia, the
first discovery being made in the bed of the Onot
stream by the explorer and prospector J. P. Alibert,
in 1850. A large boulder of this kind, weighing
over half a ton (1156 lb., or 524-5 kg.), is exhib-
ited in the Mineral Gallery of the British Museum
(Natural History). An enormous mass, weighing over
2 tons (4718 lb., or 2140 kg.), was discovered at
Jordansmiihl, Silesia, by Dr. G. F. Kunz, and is now
in the magnificent collection of jade formed by
Mr. Heber R. Bishop. Beautiful greenstone occurs
in New Zealand, particularly in the Middle Island.
The Maoris have long used it for various useful and
ornamental purposes, the most common being
indicated by their general name for the species,
punamu, axe-stone ; kawakawa is the ordinary
green variety, a fine section of which is shown on
the wall of the Mineral Gallery of the British
Museum (Natural History), while inanga, a grey
variety, and kakurangi, a pale-green and translucent
variety, are rare and highly prized.
Jadeite (Plate XXIX, Fig. 18) is by far the rarer
of the two jades, and is the choicest gem with the
JADE 263
Chinese. In composition it is a silicate of sodium
and aluminium with the formula NaAl(SiO3)2, corre-
sponding to the lithium mineral spodumene (p. 265).
It has the same toughness and greasy lustre as
nephrite, but is harder, being represented by the
symbol 7 on Mohs's scale, and thus only slightly,
if at all, softer than quartz. The other characters
are also higher; the specific gravity is about 3^34,
and the least and greatest of the principal refractive
indices are i'66 and r68, the double refraction
being biaxial and negative. The colour varies
from white to almost an emerald green, the latter
being especially prized, and often the green colour
runs in streaks through the white. Jadeite fuses
readily before the blowpipe to blebby glass, more
easily than is the case with nephrite.
The finest jadeite comes from the Mogaung
district in Upper Burma, where it is found in
boulders and also with albite in dykes in a dark-
green serpentine. The export trade to China, which
absorbs practically the whole of the output, is
exceedingly valuable, and realizes nearly as much
as the produce of the ruby mines. Jadeite is also
found in the Shensi and Yunnan provinces of China,
and in Tibet.
A few words may be said about the other jade-
like minerals. Saussurite, which is named after
H. B. de Saussure, has resulted from the decomposi-
tion of a felspar, and is nearly akin to the mineral
zoisite. It has the customary toughness of structure,
and is greenish grey to white in colour. Its specific
gravity is about 3 '2, and hardness 6£ to 7 on Mohs's
scale. It occurs near Lake Geneva. Bowenite is
264 GEM-STONES
a green serpentine (p. 289) which is found at
Smithfield, Rhode Island, U.S.A., and in New
Zealand and Afghanistan. Californite and plasma
are compact varieties of idocrase (p. 275) and
chalcedony (p. 247) respectively. Verdite is a stone
of rich green colour which is found in the form of
large boulders in the North Kaap River, South
Africa ; it is composed of green mica (fuchsite) and
some clayey matter.
Jade has of recent years been imitated in glass,
but the latter is recognizable by its vitreous lustre
and inferior hardness, and sooner or later by its
frangibility.
CHAPTER XXXIV
SPODUMENE, IOLITE, BENITOITE
SPODUMENE
(Kunzite, Hiddenite)
TILL a few years ago scarcely known out-
side the ranks of mineralogists, spodumene
suddenly leaped into notice in 1903 upon the
discovery of the lovely lilac-coloured stones (Plate
XXIX, Fig. 10) at Pala, San Diego County, Cali-
fornia; they shortly afterwards received the name
kunzite after the well-known expert in gems, Dr.
G. F. Kunz. The stones were found here in a peg-
matite dyke, and were of all shades, ranging from
pale pink to deep lilac, and at times as much as
150 carats in weight. Paler kunzite occurs with
beryl and tourmaline at Coahuila Mountain in River-
side County, California, and colourless stones have
recently come to light in Madagascar. Kunzite
is remarkable for its wonderful dichroism ; the
beautiful violet tint that springs out in one direction
comes with greater surprise because of the un-
interesting yellowish tints in other directions.
Unlike spodumene in general, kunzite is phosphor-
escent under the influence of radium.
The emerald-green variety (Plate XXIX, Fig. 1 1),
266 GEM-STONES
named hiddenite after Mr. W. E. Hidden, who
discovered in 1881 the only known occurrence, in
Alexander County, North Carolina, would no doubt
have become popular had the supply of material not
been so very limited ; few stones were found, and
the variety has never come to light elsewhere. The
colour is supposed to be due to chromic acid.
Hiddenite being also dichroic, the tint varies with
the direction. .
Spodumene is ordinarily rather a pale yellowish
in hue, and, as its name (which is derived from
<77ro8('o9, ash-coloured) suggests, is not very attractive.
Clear, lemon-yellow stones (Plate XXIX, Fig. 9) are
found in Brazil and Madagascar.
The species is interesting scientifically because it
contains the rare element lithium ; it is a silicate of
aluminium and lithium, corresponding to the formula
LiAl(SiO3)2. The double refraction is biaxial \m
character and positive in sign, the least and greatest
of the refractive indices being r66o and 1*675 > the
specific gravity is S'iS'S, and hardness 6£ to 7 on
Mohs's scale. Spodumene has an easy cleavage,
and the cut stones call therefore for careful handling,
lest they be flawed or fractured. Two faceted
stones, a beautiful kunzite and a fine hiddenite,
weighing 60 and 2.\ carats respectively, are ex-
hibited in the British Museum (Natural History).
lOLITE
Known also by various other names — cordierite,
dichroite, and water-sapphire (saphire <?eau*) — this
species owes its interest to the remarkable dichroism
characterizing it, the principal colours — smoky-blue
SPODUMENE, IOLITE, BENITOITE 267
and yellowish white — being in such contrast as to
be obvious to the unaided eye. The stones that
are usually worked have intrinsically a smoky-blue
colour, and are found in watenvorn masses in the
river-gravels of Ceylon, whence is the origin of the
name water-sapphire. lolite, from LOV, violet, and
Xi#o9, stone, refers to the colour ; cordierite is named
after Cordier, a French geologist, who first studied
the crystallography of the species ; and dichroite, of
course, alludes to the most prominent character of
the species.
lolite is a silicate of aluminium and of magnesium
and iron corresponding to the formula H2(Mg,Fe)4
Al8Si10O37. The double refraction is small in
amount, biaxial in character, and negative in sign,
the least and greatest of the refractive indices being
1-543 and 1*55 *J th6 specific gravity is 2*63, and
hardness 7 on Mohs's scale. lolite, if used, is
worked and polished; it is seldom faceted. A
large worked piece, weighing 177 grams, which was
formerly in the Hawkins Collection, is exhibited in
the British Museum (Natural History).
BENITOITE
The babe among gem-stones, benitoite first saw
the light of day a few years ago, early in 1907.
It occurs with the rare mineral neptunite, which was
previously known only from Greenland, in narrow
veins of natrolite in Diablo Range near the head-
waters of the San Benito River, San Benito County,
California. Despite careful search the species has
not been found except within the original restricted
area. To science it is interesting both because of
268 GEM-STONES
its composition, a silico-titanate of barium, corre-
sponding to the formula BaTiSi3O9, and because its
crystals belong to a class of crystalline symmetry
which has hitherto not been represented among
minerals. The double refraction is uniaxial, and
since the ordinary index of refraction is 1757 and
the extraordinary 1-804, it is positive in sign and
large in amount, namely, O'O47. The stones are
characterized by strong dichroism, the colour corre-
sponding to the ordinary ray being white, and to the
extraordinary greenish blue to indigo depending
upon the tint of the stone. To obtain the best
effect the stone must therefore be cut with the table-
facet parallel to the crystallographic axis. The
specific gravity is 3'65, and hardness 6| on Mohs's
scale. When first discovered the species was
supposed to be sapphire, and many stones were cut
and sold as such. It is, however, much softer than
sapphire, and is readily distinguished by its optical
characters, since it possesses greater double refraction
and of differing sign, so that, when tested with the
refractometer, the shadow-edge corresponding to the
lower index of refraction remains fixed in the case of
of benitoite, whereas the contrary happens with
sapphire. Benitoite also, unlike sapphire, fuses
easily to a transparent glass. Its blue colour,
which is supposed to be due to a small amount of
free titanic acid present, appears to be stable.
Several stones as large as I £ to 2 carats in weight
have been found. The largest of all, perfectly flaw-
less, weighs just over 7 carats, and is remarkable
because it is about three times the next largest in
point of weight ; it is the property of Mr. G. Eacret,
of San Francisco.
CHAPTER XXXV
EUCLASE, PHENAKITE, BEItYLLONITE
EUCLASE
THIS species comes near beryl in chemical com-
position, being a silicate of aluminium and
beryllium corresponding to the formula Be(AlOH)
SiO4, and closely resembles aquamarine in colour
and appearance when cut. Owing to the rarity of
the mineral good specimens command high prices
for museum collections, and it is seldom worth while
cutting it for jewellery. It derives its name from its
easy cleavage, tv easily, and /eXatri? fracture. The
double refraction is biaxial in character and positive
in sign, the least and greatest of the refractive
indices being 1-651 and 1-670 respectively; the
specific gravity is 3*07, and the hardness 7| on
Mohs's scale. The colour is usually a sea-green,
but sometimes blue. Euclase occurs with topaz at
the rich mineral district of Minas Novas, Minas
Geraes, Brazil, and has also been found in the Ural
district, Russia.
PHENAKITE
Another beryllium mineral, phenakite owes its
name to the frequency with which it has been
mistaken for quartz, being derived from </>tWf,
269
270 GEM-STONES
deceiver. The clear, colourless crystals, somewhat
complex in form, have at times been cut, but they
lack 'fire,' and despite their brilliant lustre meet
with little demand. The composition is a silicate of
beryllium corresponding to the formula Be2SiO4.
The double refraction is uniaxial, and since the
ordinary, 1*652, is less than the extraordinary index,
1-667, it is positive in sign; the specific gravity is
2'99, and the hardness is almost equal to that of
topaz, being about 7 £ to 8 on Mohs's scale.
Fine stones have long been known near Ekaterin-
burg in the Ural Mountains, and have recently been
discovered in Brazil.
BERYLLONITE
As its name suggests, this mineral also contains
beryllium, being a soda phosphate corresponding to
the formula NaBePO4. Clear, colourless stones,
which occur at Stoneham, Maine, U.S.A., have been
cut, but the lack of ' fire,' the easy cleavage, and
comparative softness, the symbol being 5| on Mohs's
scale, unfit it for use in jewellery. The double re-
fraction is biaxial in character and negative in sign,
the least and the greatest of the refractive indices
being i'553 and 1-565 respectively.
CHAPTER XXXVI
ENSTATITE, DIOPSIDE, KYANITE, ANDALUSITE,
IDOCRASE, EPIDOTE, SPHENE, AXINITE,
PREHNITE, APATITE, DIOPTASE
ENSTATITE
(' Green Garnet ')
THE small green stones which accompany
the diamond in South Africa have been cut
and put on the market as ' green garnet.' They
are, however, in no way connected with garnet, but
belong to a mineral species called enstatite, which is
a silicate of magnesium corresponding to the formula
MgSiO3 ; the green colour is due to a small amount
of ferrous oxide which replaces magnesia. The
double refraction is biaxial in character and positive
in sign, the least and greatest of the refractive
indices being 1*665 and 1*674 respectively; the
specific gravity ranges from 3*10 to 3*13, and the
hardness is only about S| on Mohs's scale. The
dichroism is perceptible, the twin-colours being
yellowish and green, and, as usual, is more pro-
nounced the deeper the colour of the stone. There
is also a good cleavage in two different directions.
With increasing percentage amount of iron
enstatite passes into hypersthene. The colour
272 GEM-STONES
becomes a dark brownish green, and an increase
takes place in the physical constants, the least and
greatest of the refractive indices attaining to 1*692
and i '7° 5 respectively, and the specific gravity
ranging from 3*4 to 3*5. Hypersthene is never
sufficiently transparent for faceting, but when
spangled with small scales of brookite it is sometimes
cut en cabochon.
The name enstatite is derived from ei/o-Tar^<?, an
opponent, referring to the infusibility of the mineral
before the blowpipe, and hypersthene comes from
t>7re/3<7#ei/o9, very tough.
An altered enstatite, leek-green in colour and
with nearly the composition of serpentine (p. 289),
has been cut en cabochon. It has much lower
specific gravity, only 2*6, and lower hardness, 3| to
4 on Mohs's scale. It is named bastite from Baste
in the Harz Mountains, where it was first discovered.
DlOPSlDE
This species, which is also known as malacolite
and alalite, provides stones of a leaf-green colour
which have occasionally been cut. It is a silicate
of calcium and magnesium corresponding to the
formula MgCa(SiO3)2, but usually contains in place
of magnesia some ferrous oxide, to which it owes its
colour; with increase in the percentage amount of
iron the colour deepens and the physical constants
change. The double refraction is large in amount,
0*028, biaxial in character, and positive in sign.
The least and greatest of the refractive indices
corresponding to the stones suitable for jewellery
range about 1-671 and 1*699 respectively, but they
KYANITE 273
may be as high as 1732 and 1750 in the two
cases. The specific gravity varies from 3*20 to 3 '3 8,
and the hardness from 5 to 6 on Mohs's scale.
Dichroism is noticeable in deep-coloured stones, but
is not very marked.
The name diopside comes from £19, double, and
0^9, appearance, in allusion to the effect resulting
from the double refraction ; malacolite is derived
from /LiaXa#o9, soft, because the mineral is softer than
the felspar associated with it ; and alalite is named
after the principal locality, Ala Valley, Piedmont,
Italy.
KYANITE
Kyanite, also known as disthene, is interesting for
two reasons. Its structure is so grained in character
that the hardness varies in the same stone from 5 to
7 on Mohs's scale ; it can therefore be scratched by
a knife in some directions, but not in others (p, 79).
It has the same chemical composition as andalusite,
both being silicates of aluminium corresponding to
the formula Al2SiO6, but possesses very different
physical characters, a fact which shows how large a
share the molecular grouping has in determining the
aspect of crystallized substances. It is biaxial with
small negative double refraction, the least and
greatest of the refractive indices being 172 and
173 respectively; the specific gravity is 3*61. It
occurs in sky-blue prismatic crystals, whitish at the
edges, in schist near St. Gothard, Switzerland. It is
seldom cut.
Kyanite is derived from its colour, tcvavos blue,
and disthene, from its variable hardness, Bl<s, twice,
and aOevos, strong.
18
274 GEM-STONES
ANDALUSITE
Andalusite bears no resemblance whatever to
kyanite, although, as has been stated above, the
composition of the two species is essentially the
same. It is usually light bottle-green in colour,
and more rarely brown and reddish. Its extreme
dichroism is its most remarkable character, the twin
colours being olive-green and red. The reddish
gleams that are reflected from the interior are in
sharp contrast with the general colour of the stone,
and impart to it a weird effect (Plate XXIX, Fig. 1 5).
Cut stones are often confused with tourmalines, and
can, indeed, only be distinguished from the latter
with certainty by noting on the refractometer the
smaller amount of double refraction and the differ-
ence in its character. The least and greatest of the
refractive indices are 1*632 and r643 respectively,
and the double refraction, O'Oi I, about half that of
tourmaline, is biaxial and negative; the specific
gravity is 3*18, and hardness 7^ on Mohs's scale.
Good stones are found at Minas Novas, Minas
Geraes, Brazil, and in the gem-gravels of Ceylon.
It was first known from the province of Andalusia,
Spain, whence is the origin of its name.
IDOCRASE
( Vesuvianite, Calif ornite)
Idocrase, also known as vesuvianite, is occasionally
found in the form of transparent, leaf-green, and
yellowish-brown stones which, when cut, may be
mistaken for diopside and epidote respectively, but
are distinguishable from both by the extreme small-
EPIDOTE 275
ness of their double refraction. Californite is a
compact variety which has all the appearances of a
jade; its colour is green, or nearly colourless with
green streaks.
In composition idocrase is a silicate of aluminium
and calcium, the precise formula of which is un-
certain, but may be —
(Ca)Mn,Mg,Fe)2[(Al,Fe)(OH>F)]Si207.
The double refraction, which is uniaxial in character
and negative in sign, may be less than crooi, and
never exceeds o-oo6, so that it is not easily detected
with the refractometer, even in sodium light The
refractive indices vary enormously in value, from
1*702 to I "j 26 for the ordinary, and from 1-706
to 1732 for the extraordinary ray. The specific
gravity varies from 3*35 to 3*45, and the hardness
is about 6^ on Mohs's scale.
The name idocrase, from etSo?, form, and icpcUris,
mixture, was assigned to the species by Haiiy,
but his reasons have little meaning at the present
day. The other names are taken from the localities
where the species and the variety were first discovered.
Bright, green crystals come from Russia, and
also from Ala Valley, Piedmont, and Mount Vesu-
vius, Italy. Californite is found in large masses in
Siskiyon and Fresno Counties, California.
EPIDOTE
(Pistactte)
Epidote often possesses a peculiar shade of
yellowish green, similar to that of the pistachio-nut —
hence the origin of its alternative name — which is
276 GEM-STONES
unique among minerals, though scarcely pleasing
enough to recommend it to general taste. Its ready
cleavage renders it liable to flaws; nevertheless, it
is occasionally faceted. The name epidote, from
eVtSoo-i?, increase, was given to it by Haiiy, but not
on very precise crystallographical grounds.
In composition this species is a silicate of calcium
and aluminium, with some ferric oxide in place of
alumina, corresponding to the complex formula,
Ca2(Al,Fe)2[(Al,Fe)OH](SiO4)3. It occurs in mono-
clinic, prismatic crystals richly endowed with
natural faces. The colour deepens with increase
in the percentage amount of iron, and the stones
become almost opaque. The double refraction is
large in amount, 0*031, biaxial in character, and
negative in sign. The dichroism is conspicuous in
transparent stones, the twin-tints corresponding to
the principal optical directions being green, brown,
and yellow. The values of the least and greatest
of the refractive indices given by transparent stones
are 1*735 and 1*766 respectively; the specific
gravity varies from 3*25 to 3*50, and the hardness
from 6 to 7 on Mohs's scale.
Transparent crystals have come from Knappen-
wand, Untersulzbachtal, Salzburg, Austria; Traver-
sella, Piedmont, Italy ; and Arendal, Nedenas,
Norway. Magnificent, but very dark, crystals were
discovered about ten years ago on Prince of Wales
Island, Alaska.
SPHENE
(Titanite)
The clear, green, yellow, or brownish stones
provided by this species would be welcomed, in
SPHENE 277
jewellery because of their brilliant and almost
adamantine lustre, but, unfortunately, they are too
soft to withstand much wear, the hardness being
only 5i on Mohs's scale. In composition sphene
is a silico-titanate of calcium corresponding to the
formula CaTiSiO6, and in this respect comes near
the recently discovered gem-stone, benitoite. The
refractive indices lie outside the range of the re-
fractometer, the values of the least and the greatest
of the refractive indices varying from i'888 and
i '9 1 7 to i '9 1 4 and 2*053 respectively. It is to
this high refraction that it owes its brilliant lustre.
The double refraction, which is biaxial in character
and positive in sign, is so large that the apparent
doubling of the opposite edges of a cut stone when
viewed through one of the faces is obvious to the
unaided eye (cf. p. 41). Cut stones have ad-
ditional interest on account of the vivid dichroism
displayed, the twin-tints, colourless, yellow, and
reddish yellow, corresponding to the three principal
optical directions, being in strong contrast. The
specific gravity ranges from 3*35 to 3*45. The
negative test with the refractometer (cf. p. 26), the
softness, and the large amount of double refraction
suffice to distinguish this species from gem-stones
of similar appearance.
The name sphene, from a-Qijv, wedge, alludes to
the shape of the natural crystals. The alternative
name is obviously due to the fact that the species
contains titanium.
Good stones have come from the St. Gothard
district, Switzerland.
278 GEM-STONES
AXINITE
Called axinite from the shape of its crystals —
], axe — this species supplies small, clear, clove-
brown, honey-yellow, and violet stones which can
be cut for those who care for a stone out of the
ordinary. The composition is a boro-silicate of
aluminium and calcium, with varying amounts of
iron and manganese, corresponding to the formula
(Ca,Fe)3Al2(B.OH)Si4O15. Axinite is interesting on
account of its strong dichroism, the twin-tints corre-
sponding to the principal optical directions being
violet, brown, and green. The double refraction is
biaxial in character and negative in sign, the least
and greatest of the refractive indices being 1*674
and 1*684; the specific gravity is 3'2 8, and hard-
ness about 6-g- to 7, or rather under that of quartz.
The best examples have been found at St.
Cristophe, Bourg d'Oisans, in the Dauphind, France.
Violet axinite is a novelty that has come within
recent years from Rosebery, Montagu County,
Tasmania.
PREHNITE
This species, which is named after its discoverer,
Colonel Prehn, is found in nodular, yellow and
oil-green stones, of which the latter have very
occasionally been cut. It is a little soft, the
hardness being only 6 on Mohs's scale. The
double refraction is large in amount, 0*03 3, biaxial
in character, and positive in sign, the least and the
greatest of the refractive indices being I '6 1 6 and
r649 respectively; the specific gravity varies
from 2'8i to 2-95. In composition prehnite is a
APATITE 279
silicate of aluminium and calcium corresponding
to the formula H2Ca2Al2(SiO4)3.
The best material has been found at St.
Cristophe, Bourg d'Oisans, Dauphine", France.
APATITE
This interesting mineral is found occasionally
in attractive green, blue, or violet stones, but is
unfortunately too soft for extensive use in jewellery,
the hardness being only 5 on Mohs's scale. In
composition it is a fluo - chloro - phosphate of
calcium, corresponding to the formula Ca4[Ca(F,Cl)]
(PO4)3. When pure, it is devoid of colour, the
tints being due to the presence of small amounts
of tinctorial agents. The double refraction is
uniaxial in character and negative in sign, the
ordinary index being r642 and the extraordinary
1*646; the specific gravity varies from 3-i7 to
3*23. The dichroism is usually feeble, but some-
times is strong ; for instance, in the stones from
the Burma ruby mines (yellow, blue-green). A
cut stone might be mistaken for tourmaline, but
is distinguished by its softness, or, when tested on
the refractometer, by its inferior double refraction.
It received its name from a-jrardeiv, deceive, because
it was wrongly assigned to at least half a dozen
different species in early days. Moroxite is a name
sometimes given to blue-green apatite.
Beautiful violet stones are found at Ehrenfried-
ersdorf, Saxony; Schlaggenwald, Bohemia; and
Mount Apatite, Auburn, Androscoggin County,
Maine, U.S.A. ; and blue stones come from Ceylon.
280 GEM-STONES
DlOPTASE
Though of a pretty, emerald-green colour, dioptase
has never been found in large enough crystals for
gem purposes, and it is, moreover, rather soft, the
hardness being only 5 on Mohs's scale, and has
an easy cleavage. In composition it is a hydrous
silicate of copper corresponding to the formula
CuH2SiO4. The double refraction, which is
large in amount, is uniaxial in character, and
positive in sign, the ordinary refractive index
being r66/ and the extraordinary i"J2^. Its
colour and softness distinguish it from peridot or
diopside, which have about the same refractivity.
The name was assigned to the species by HaUy,
from Bia, through, and oTrro/Aat, see, because the
cleavage directions were distinguishable by looking
through the stone.
Dioptase has been found near Altyn-Ttibe in
the Kirghese Steppes, at Rezbanya in Hungary,
and Copiapo in Chili, and at the mine Mindouli,
near Comba, in the French Congo.
CHAPTER XXXVII
CASSITERITE, ANATASE, PYRITES, HEMATITE
CASSITERITE
THOUGH usually opaque, this oxide of tin,
corresponding to the formula SnO2, has
occasionally, but very rarely, been found in small,
transparent, yellow and reddish stones suitable
for cutting. The lustre is adamantine. The
refraction is uniaxial in character and positive in
sign, the ordinary index being 1*997 and extra-
ordinary 2-093. The specific gravity is high,
ranging from 6'8 to yi. The hardness is on the
whole less than that of quartz, being about 6 to 7
on Mohs's scale.
ANATASE
This mineral, which is one of the three crys-
tallized forms of titanium oxide, TiO2, occurs
often in small, brown, transparent stones which
occasionally find their way into the market. The
lustre is adamantine. The refraction is uniaxial
in character and negative in sign, the extraordinary
index being 2-493 and ordinary 2-554. The
specific gravity varies from 3-82 to 3-95, and the
hardness is about 5 i to 6 on Mohs's scale.
282 GEM-STONES
PYRITES, HEMATITE
These two metallic minerals were employed in
ancient jewellery. The former, sulphide of iron,
FeS2, is brass-yellow in colour, and has a specific
gravity 5*2, and hardness 6\ on Mohs's scale. It
is found, when fresh, in brilliant cubes. The latter,
oxide of iron, Fe2O3, has a black metallic lustre,
but, when powdered, is red in colour — a mode of
distinguishing it from other minerals of similar
appearance. Its specific gravity is 5^3, and hard-
ness 6£ on Mohs's scale. In modern times it has
been cut in spherical form to imitate black pearls,
but can easily be recognized by its greater density
and hardness. Hematite is used for signet stones,
often with an intaglio engraving.
CHAPTER XXXVIII
OBSIDIAN, MOLDAVITE
TWO forms of natural glass have been em-
ployed for ornamental purposes. Obsidian
results from the solidification without crystallization
of lava, and corresponds in composition to a granite.
The structure is seldom clear and transparent, and
usually contains inclusions or streaks. The colour
is in the mass jet-black, but smoky in thin frag-
ments, and occasionally greenish. Its property of
breaking with a keen cutting edge, in the same
way as ordinary glass, rendered it of extreme
utility to primitive man, who was ignorant of the
artificial substance. The refraction is, of course,
single, and the refractive index approximates to
1-50. The specific gravity varies from 2*3 to 2-5.
The hardness is 5 on Mohs's scale, the same as
ordinary glass.
Obsidian is obtained wherever there has been
volcanic activity. Vast mines of great antiquity
exist in the State of Hidalgo, Mexico.
Moldavite, which differs in no respect from
ordinary green bottle-glass, is of interest on account
of its problematical origin. Its occurrence in
various parts of Bohemia and Moravia cannot be
explained as the result of volcanic agency. It
may possibly be the product of old and forgotten
a83
284 GEM-STONES
glass factories which at one time existed on the
site. Even meteorites have been suggested as
the source. The physical characters are the same
as those of ordinary glass : refraction single, index
1*51; specific gravity 2-50 and hardness 5-5- on
Mohs's scale. Moldavite also passes under the
names of bottle-stone, or water - chrysolite. A
natural glass of the same character has been found
in water-worn fragments in Ceylon, and has been
sold as peridot, which it resembles in colour, but is
readily distinguished from it by its very different
physical properties.
PART II— SECTION C
ORNAMENTAL STONES
CHAPTER XXXIX
FLUOR, LAPIS LAZULI, SODALITE, VIOLANE,
RHODONITE, AZURITE, MALACHITE,
THULITE, MARBLE, APOPHYLLITE, CHRY-
SOCOLLA, STEATITE OR SOAPSTONE,
MEERSCHAUM, SERPENTINE
SPACE will not permit of more than a few
words concerning the more prominent of the
numerous mineral species which are employed for
ornamental purposes in articles of virtu or in archi-
tecture, but which for various reasons cannot take
rank as gem-stones.
Fluor, a beautiful mineral which is found in its
greatest perfection in England, has enjoyed well-
deserved popularity when worked into vases or other
articles. The finest material, deep purple in colour,
known as ' Blue John/ came from Derbyshire, but
the supply is now exhausted. The crystallized
examples, from Durham, Devonshire, and Cornwall,
form some of the most attractive of museum
specimens. The crystals take the shape of cubes,
often twinned, and have an easy octahedral cleavage.
286 GEM-STONES
The refraction is single, the index being i'433.
Fluor is noted for its property of appearing of
differing colour by reflected and transmitted light,
and the phenomenon is in consequence known as
fluorescence. The specific gravity is 3*18, and the
hardness 4 on Mohs's scale. Owing to its low
refraction and softness, fluor is not suitable for
jewellery. Clear colourless material is in demand
for particular lenses of microscope objectives.
The lovely blue stone known as lapis lazuli has
since the earliest times been applied to all kinds of
decorative purposes, for mosaic and inlaid work and
as the material for vases, boxes, and so on, and was
the original sapphire of the ancients. When ground
to powder it furnishes a fine blue paint, but it has
now been entirely superseded for this purpose by an
artificial product. Although to the eye so homo-
geneous and uniform in structure, lapis lazuli has
been shown by microscopic examination to be
composed of calcite coloured by three blue minerals
in varying proportions. All three belong to the
cubic class of symmetry, and are mainly soda
aluminium silicates in composition ; their hardness
varies from 5 to 6 on Mohs's scale. Lazurite,
Na4(NaS3.Al)Al2Si3O12, has specific gravity varying
from 2*38 to 2'45, and hardness about 5 to 5^;
hauynite, (Na2,Ca)2(NaSO4,Al)Al2Si3O12, is about
the same in specific gravity, 2*4 to 2'5, but slightly
harder, 5£ to 6 ; while sodalite, Na4(AlCl)Al2Si3O12,
is the lightest in density, 2-14 to 2-30, with hardness
5£ to 6, and has a refractive index I '4 8 3.
By far the oldest mines are in the Badakshan
district of Afghanistan, a few miles above Firgamu
in the valley of the Kokcha, a branch of the Oxus,
SODALITE, VIOLANE, RHODONITE 287
where ruby and spinel are found. It is also
found at the southern end of Lake Baikal, Siberia,
and in the Chilian Andes.
Sodalite occurs in beautiful blue masses at
Dungannon, Hastings County, Ontario, Canada,
and at Litchfield, Maine, U.S.A. They make
excellent polished stones.
Violane, a massive, dark violet-blue diopside from
San Marcel, Piedmont, Italy, also makes a handsome
polished stone.
Rhodonite, silicate of manganese, MnSiO3,
possesses a fine red colour, and makes an attractive
stone when cut and polished. It has very slight
biaxial double refraction, the refractivity being about
173 ; the specific gravity is 3-6, and hardness 6.
It is found in large masses near Ekaterinburg in the
Ural Mountains, and is quarried as an ornamental
stone.
Both the copper carbonates, azurite or chessylite,
and malachite, make effective polished stones. The
latter is also worked into various ornamental objects ;
it occurs in fibrous masses, the grained character of
which look well in the polished section. Its colour
is a bright green, to which it owes its name, from
fj,a\a,Kr), mallows. Its composition is represented by
the- formula CuCO3.Cu(OH)2, and it is the more
stable form, since azurite is frequently found altered
to it. It has biaxial double refraction, and the
indices are about r88 ; the specific gravity is 4*01,
and hardness about 3^ to 4 on Mohs's scale. It is
found in large masses at the copper mines of Nizhni
Tagilsk in the Ural Mountains, where it is mined as
an ornamental stone ; it also accompanies the copper
ores in many parts of the world, for instance Cuba,
288 GEM-STONES
Chili, and Australia. Azurite, so called on account
of its beautiful blue colour, is rarer, but, unlike
malachite, is generally in the form of crystals.
Beautiful specimens have come from Chessy, near
Lyons, France, and Bisbee, Arizona, U.S.A. The
composition corresponds to the formula 2CuCO3,
Cu(OH)2. The specific gravity is 3'8o, and hard-
ness about 3 1 to 4.
Chrysocolla occurs in blue and bluish-green
earthy masses, with an enamel-like texture, which in
some instances can be worked and polished. Being
the result of the decomposition of copper ores, it
varies considerably in hardness, ranging from 2 to
4 on Mohs's scale. Its composition approaches to
the formula CuSiO3.2H2O, but it invariably contains
impurities. It is very light, the density being only
about 2-2.
Steatite, or soapstone, is a massive foliated sili-
cate of magnesium corresponding to the formula
H2Mg3Si4O12) which is one of the softest of mineral
substances, representing the degree I on Mohs's
scale, but in massive pieces is harder owing to the
intermixture of other substances with it. It has a
peculiar greasy feeling to the touch, due to its softness.
The specific gravity is about 275. The Chinese carve
images out of the yellowish and brownish pieces.
Meerschaum, a silicate of magnesium corre-
sponding to the formula H4Mg2Si3O10, is familiar
to every smoker as a material for pipe-bowls. It
is very light, the specific gravity being only 2'O,
and soft, the hardness being about 2 to 2 £ on Mohs's
scale. When found, it is pure white in colour, and
answers to its name, a German word signifying sea~
foam, It comes from Asia Minor.
SERPENTINE 289
Serpentine has been largely used for decorative
purposes, as well as for cameos and intaglios, and
formed most of the famous ' verde antique.' Being
the result of the decomposition of other silicates it
varies enormously in appearance and characters, but
the most attractive stones are a rich oil-green in
colour and resemble jade. The composition approxi-
mates to the formula H4Mg3Si2O9, but it invariably
contains other elements. The hardness varies from
2\ to 4 on Mohs's scale, according to the minerals
contained in the stone ; the specific gravity is about
2 '60 and the refractivity i'57O.
The beautiful rose-red stone, thulite, makes a
handsome decorative stone. It has nearly the same
composition as epidote (p. 275), and like it has
strong dichroism, the principal colours being yellow,
light rose, and deep rose. The colour is due to
manganese. Its refractive index is about i'7o,
specific gravity 3' 12, and hardness 6 to 6£ on
Mohs's scale ; it possesses an easy cleavage. Fine
specimens come from Telemark, Norway, and it is
therefore called after the old name for Norway,
Thule.
Marble is a massive calcite, carbonate of lime,
with the formula CaCO3. When pure it is white,
but it is usually streaked with other substances
which impart a pleasing variety to its appearance.
It is always readily recognized by the immediate
effervescence set up when touched with a drop of
acid. Calcite is highly doubly refractive (cf. p. 40),
the extraordinary index being 1-486, and ordinary
1-658, a difference of 0*172 ; the specific gravity is
2*71, and hardness 3 on Mohs's scale. Lumachelle,
or fire-marble, is a limestone containing shells from
19
290 GEM-STONES
which a brilliant, fire-like chatoyancy is emitted
when light is reflected at the proper angle. It
sometimes resembles opal-matrix, but is easily dis-
tinguished by its lower hardness and by its effer-
vescent action with acid. Choice specimens come
from Bleiberg in Carinthia, and from Astrakhan.
Apophyllite has not many characters to commend
it, being at the best faintly pinkish in colour, and
always imperfectly transparent. It is a hydrous
silicate of potassium and calcium with the complex
formula (H,K)2Ca(SiO3)2.H2O. Its refractivity is
about 1*535, specific gravity 2*5, and hardness 4| on
Mohs's scale ; it possesses an easy cleavage. It
occurs in the form of tetragonal crystals at Andreas-
berg in the Harz Mountains, and in the Syhadree
Mountains, Bombay, India
PART II— SECTION D
ORGANIC PRODUCTS
CHAPTER XL
PEARL, CORAL, AMBER
A LTHOUGH none of the substances considered
A\ in this chapter come within the strict defini-
tion of a stone, since they are directly the result of
living agency, yet pearl at least cannot be denied
the title of a gem. Both pearl and coral contain
calcium carbonate in one or other of its crystallized
forms, and both are gathered from the sea ; but
otherwise they have nothing in common. Amber
is of vegetable origin, and is a very different
substance.
PEARL
From that unrecorded day when some scantily
clothed savage seeking for succulent food opened an
oyster and found to his astonishment within its shell
a delicate silvery pellet that shimmered in the light
of a tropical sun, down to the present day, without
intermission, pearl has held a place all its own in the
rank of jewels. Though it be lacking in durability,
its beauty cannot be disputed, and large examples,
292 GEM-STONES
perfect in form and lustre, are sufficiently rare to
tax the deepest purse.
The substance composing the pearl is identical
with the iridescent lining — mother-o'-pearl or nacre,
as it is termed — of the shell. Tortured by the
intrusion of some living thing, a boring parasite,
a worm, or a small fish, or of a grain of sand or
other inorganic substance, and without means to free
itself, the mollusc perforce neutralizes the irritant
matter by converting it into an object of beauty
that eventually finds its way into some jewellery
cabinet. Built up in a haphazard manner and not
confined by the inexorable laws of intermolecular
action, a pearl may assume any and every variety
of shape from the regular to the fantastic. It may
be truly spherical, egg- or pear-shaped — pear-drops
or pear-eyes, as they are termed — or it may be
quite irregular — the so-called baroque or barrok
pearls. The first is the most prized, but a well-
shaped drop-pearl is in great demand for pendants
or ear-rings. The colour is ordinarily white, or
faintly tinged yellowish or bluish, and somewhat
rarely, salmon-pink, reddish, or blackish grey.
Perfect black pearls are valuable, but not as costly
as th&. finest of the white. Though not transparent,
pearl is to a varying extent translucent, and its
characteristic lustre — ' orient ' in the language of
jewellery — is due to the same kind of interaction of
light reflected from different layers that has been
remarked upon in the case of opal and certain other
stones. The translucency varies in degree, and some
jewellers speak of the ' water ' of pearls just as in
the case of diamonds. If a pearl be sliced across
the middle and the section be examined under the
PEARL, CORAL, AMBER 293
microscope, it will be seen that the structure consists
of concentric shells and resembles that of an onion.
These shells are alternately composed of calcium
carbonate in its crystallized form, aragonite, and of
a horny organic matter known as conchiolin, and
they evidently represent the result of intermittent
growth. Because of their composite character,
pearls have a specific gravity ranging from 2-65
to 2^69 — 2*84— 2-89 in the case of pink pearls —
which is appreciably less than that of aragonite,
2'94 : the hardness is about the same, namely, 3^ to
4 on Mohs's scale. That the arrangement of the
mineral layers is approximately parallel is evinced
by the distinctness of the shadow-edges shown on
examination with the refractometer. Pearls require
very careful handling, both because they are com-
paratively soft and therefore apt to be scratched,
and because they are chemically affected by acids,
and even by the perspiration from the skin. Acids
attack only the calcium carbonate, not the organic
matter ; the well-known story therefore of Cleopatra
dissolving a valuable pearl in vinegar, which is
moreover, too weak an acid to effect the solution
quickly, must not be accepted too literally. Pearls
are not cut like stones, and therefore as soon as the
precious bloom has once gone, nothing can be done
to revive it. Attempts are sometimes made in the
case of valuable pearls to remove the dull skin and
lay bare another iridescent layer underneath, but
the operation is exceedingly delicate. Even with
the best of care pearls must in process of time
perish owing to the decay of the organic constituent.
Pearls that have been discovered in ancient tombs
crumbled to dust at a touch, and those formerly in
294 GEM-STONES
ancient rings have vanished or only remain as a
brown powder, while the garnets or other stones set
with them are little the worse for the centuries that
have passed by.
The largest known pearl was at one time in the
famous collection belonging to the banker, Henry
Philip Hope. Cylindrical in form, with a slight
swelling at one end, it measures 50 mm. (2 inches)
in length, and 115 mm. (4! inches) in circumference
about the thicker, and 83 mm. (3^ inches) about the
thinner end, and weighs 454 carats. About three-
quarters of it is white in colour with a fine ' orient,'
and the remainder is bronze in tint. It is valued at
upwards of £12,000. A large pearl, 300 carats in
weight, is in the imperial crown of the Emperor of
Austria, and another, pear-shaped, is in the posses-
sion of the Shah of Persia. A beautiful white India
pearl, a perfect sphere in shape, and 28 carats in
weight, is in the Museum of Zosima in Moscow ; it
is known as ' La Pellegrina.' The ' Great Southern
Cross,' which consists of nine large pearls naturally
joined together in the shape of a cross, was dis-
covered in an oyster fished up in 1886 off the beds
of Western Australia. The collection of jewels in
the famous Green Vaults at Dresden contains a
number of pearls of curious shapes.
Large pearls are sold separately, while the small
pearls known as ' seed ' pearls come into the market
bored and strung on silk in ' bunches.' The unit of
weight is the pearl grain, which is a quarter of a
carat, and the rate of price depends on the square
of the weight in grains. The rate per unit or base
varies from 6d. to 503. according to the shape and
quality of the pearl. Spherical pearls command
PL A TE XXX
PEARL, CORAL, AMBER 295
the best prices, next the pearl-drops, and lastly the
buttons ; but whatever the shape, it is imperative
that the pearl have ' orient,' without which it is
valueless. The cheaper grades of pearls are sold
by the carat.
For use in necklaces and pendants pearls are
bored with a steel drill, and threaded with silk,
an easy operation on account of their softness.
They harmonize well with diamonds. Small pearls
are often set as a frame to large coloured stones, to
which they form an admirable foil. Pearls set in
rings or anywhere where the upper half alone would
show are generally sawn in halves ; ' button ' pearls
find an extensive use in modern rings.
Any mollusc, whether of the bi-valve or the uni-
valve type, which possesses a nacreous shell, has the
power of producing pearls, but only two, the pearl-
oyster, Meleagrina margaritifera, and the pearl-
mussel, Unio margarifer, repay the cost of systematic
fishing. The outside of the shell is formed of the
horny matter called conchiolin ; while the inside is
composed of two coats, of which the outer consists
of alternate layers of conchiolin and calcium
carbonate in its crystallized form, calcite, and the
inner of the same organic matter, but with calcium
carbonate in its other crystallized form, aragonite.
The latter coat forms the nacreous lining known as
mother- o'-pearl, which is identical in consistency
with pearl, but somewhat more transparent. The
iridescence of mother-o'-pearl is due not only to the
fact that it is composed of a succession of thin
translucent layers, but also to the fact that these
layers overlap like slates on a house, and form a
series of fine parallel lines on the surface; diffrac-
296 GEM-STONES
tion therefore as well as interference of light takes
place, and a similar diffraction phenomenon is dis-
played even by a cast of the inside of the shell.
The animal has the property of secreting calcium
carbonate, which it absorbs from the sea-water, in
both its crystallized conditions as well as conchiolin.
At the outer rim it secretes conchiolin, further in
calcite, and at the very inside aragonite. The shape
and appearance of a pearl therefore depend on the
position in which the intruding substance is situated
within the shell. The most perfect pearl has been
in intermittent motion in the interior of the mollusc,
and has received successive coats according to the
position in which it happened to be. A parasite
that bores into the shell is walled up at the point of
entrance, and a wart- or blister-pearl results. The
thinner the successive coats the finer the lustre.
Pearls have even been discovered embedded in the
animal itself. The number of pearls found in
a shell depends on the number of times the living
host was compelled to seal up some irritant object,
and may vary from one up to the eighty-seven which
are said to have been found in an Indian oyster.
That an oyster thus distinguished has not led a
happy existence is testified by the distorted shape
of its shell, a clue that guides the pearl-fishers in
their search. Moreover, pearl-oysters never have
thick nacreous shells, and on the other hand molluscs
with fine mother-o'-pearl seldom contain pearls.
Beautiful white and silvery pearls are found in a
small oyster that lives at a depth of 6 to 13
fathoms (i 1—24 m.) in the Gulf of Manaar, off the
coast of Ceylon. About seven-eighths, however, of
the pearls that come into the market are obtained
PLATE XXXI
PLATE XXXll
SECTIONS OF CULTURE PEARL
FIG. I. IN THE OYSTER. FIG. 2. WHEN FINISHED
A. PEARLY DEPOSIT. B. PIECE OF MOTHER-o'-PEARL INSERTED
IN THE OYSTER. C. OUTER SHELL OF THE OYSTER. D. MOTHER-
PEARL, CORAL, AMBER 297
from a larger oyster which has its home on the
Arabian coast of the Persian Gulf. These famous
fisheries have been known since very early times.
The pearls found here are more yellowish than those
from Ceylon, but are nevertheless of excellent
quality. The pearl fisheries off the north-west coast
of Western Australia and off Venezuela are also not
unimportant, and fine black pearls have been
supplied by molluscs from the Gulf of Mexico.
The Chinese have long made a practice of
introducing into the shell of a pearl-oyster little
tin images of Buddha in order that they may be
coated with the nacreous secretion. The Japanese
have during recent years made quite an industry of
stimulating the efforts of the mollusc by cementing
small pieces of mother-o'-pearl to the interior surface
of the shell (Plate XXXII, Fig. i); these 'culture'
pearls, as they are termed, are recognizable by
examination of the back. About a year has to elapse
before a coating of a tenth of a millimetre is formed,
and another two years must pass before the thick-
ness is doubled. After removal the piece of
mother-o'-pearl, which is now coated with several
nacreous layers, is cemented to a piece of ordinary
mother-o'-pearl, and the lower portion is ground to
the usual symmetrical shape (Plate XXXII, Fig. 2).
Blister pearls are often similarly treated. In both
cases, however, the ' orient ' is deficient in quality.
The finest mother-o'-pearl is supplied by a
mollusc found in the sea near the islands lying
between Borneo and the Philippines, and fine
material is found at Shark Bay and off Thursday
Island.
298 GEM-STONES
CORAL
Coral ranks far below pearl and meets with but
limited appreciation. It is common enough in warm
seas, but the only kind which finds its way into
jewellery is the rose or red-coloured coral — the
noble coral, Corallium nobile or rubrum. It consists
of the axial skeleton of the coral polyp, and is built
up of hollow tubes fitting one within the other. The
composition is mainly calcium carbonate with a
little magnesium carbonate and a small amount of
organic matter. The former of the mineral sub-
stances is in the form of calcite, and the crystals
are arranged in fibrous form radiating at right angles
to the axis of the coral. The specific gravity varies
from 2-6 to 2'7, being slightly under that of calcite,
and the hardness is somewhat greater, being about
3! on Mohs's scale.
The best red coral is found in the Mediterranean
Sea off Algiers and Tunis in Africa, and Sicily and
the Calabrian Coast of Italy. The industry of
shaping and fashioning the coral is carried on
almost entirely in Italy. Coral is. usually cut into
beads, either round or egg-shaped, and used for
necklaces, rosaries, and bracelets. The best quality
fetches from 2os. to 303. per carat.
AMBER
This fossil resin, yellow and brownish-yellow in
tint, finds an extensive use as the material for
mouthpieces of pipes, cigar and cigarette-holders,
umbrella-handles, and so on, and is even locally cut
for jewellery, although its extreme softness, its hard-
PEARL, CORAL, AMBER 299
ness being only 2| on Mohs's scale, quite unfits it for
such a purpose. It is only slightly denser than
water, the specific gravity being about i'io. Since
the structure is amorphous the refraction is single,
the index being about i'54O. Amber, being a very
bad conductor of heat, is perceptibly warm to the
touch. Its property of becoming electrified by
friction attracted early attention, and from the
Greek name for it, rj\eKrpov, is derived our word
electricity.
Amber is washed up by the sea off the coasts of
Sicily and Prussia, and of Norfolk and Suffolk in
England. The finest examples, which are picked
up off the shore of Catania in Sicily, are distin-
guished by a fine bluish fluorescence, resembling
that seen in lubricating oil ; such pieces command
good prices.
A recent resin, pale yellow in colour, known as
kauri-gum, is found in New Zealand, where it is
highly valued.
TABLES
TABLE I
Chemical Composition of Gem- Stones
(a) ELEMENTS —
Diamond C
(i>) OXIDES—
Corundum A12OS
Quartz SiO2
Chalcedony SiO8
Opal . SiO2.nII2O
(c) ALUMINATES—
Spinel MgAl2O4
Chrysoberyl BeAl2O4
(rf) SILICATES —
Phenakite Be2SiO4
Dioptase H2CuSiO4
Peridot Mg2SiO4
Zircon ZrSiO4
Enstatite MgSiO3
Diopside CaMg(SiO3)2
- Nephrite CaMg,(SiO,)4
Sphene CaTiSiO5
Benitoite BaTiSisO9
Andalusite ...... Al(AlO)SiO4
Kyanite (AlO)2SiO3
Topaz [Al(F,OH)],Si04
Epidote . . . Ca,(Al,Fe)2(AlOH)(SiO4)3
Euclase Be(AlOH)SiO4
Prehnite H2Ca2Al,(SiO4)3
lolite H2(Mg,Fe)4Al8Si100S7
TABLES 301
SILICATES — continued
Hessonite ...... Ca3Al2(SiO4)3
Pyrope ...... MgsAl2(Si04)3
Almandine ...... Fe3Al2(SiO4)3
Andradite ...... Ca^jFe^SiOJ,,
Beryl ....... Be3Al2(SiO3)6
Spodumene ...... LiAl(SiO3)j
Jadeite ...... NaAl(SiO8)2
Moonstone ...... KAlSi3O8
Tourmaline / i*iOs.3B/>,.(9-*)[(Al,Fe)A].3*[(F*
M
Axinite ..... HCa3Al2B(SiO4)4
f(Ca,Mn,Mg,Fe)2
\[(Al,Fe)(OH,F)]Sia07
(e) PHOSPHATES —
Beryllonite ...... NaBePO4
Apatite ..... Ca6(F,Cl)(P04)3
Turquoise . . . CuOH.6[Al(OH)2].H6.(PO4)4
TABLE II
Colour of Gem-Stones
Colourless and White. — Diamond, corundum (white
sapphire), topaz, quartz (rock-crystal), zircon (when
'fired'), moonstone; rarely beryl, tourmaline;
among the less common species, phenakite,
spodumene (colourless kunzite), beryllonite.
Yellow. — Diamond, topaz, corundum (yellow sapphire),
quartz (citrine, Scotch or occidental topaz), tourma-
line, zircon, sphene, spodumene, beryl.
Pink and Lilac. — Corundum (pink sapphire), spinel
(balas-ruby), tourmaline (rubellite), topaz (usually
when 'fired'), spodumene (kunzite), beryl (mor-
ganite), quartz (rose-quartz).
Red. — Corundum (ruby), garnet (pyrope, almandine),
spinel (balas-ruby), tourmaline (rubellite), zircon,
opal (fire-opal).
302
GEM-STONES
Green. — Beryl (emerald, aquamarine), peridot, cor-
undum, tourmaline, chrysoberyl (including alex-
andrite), zircon, garnet (demantoid) ; among less
common species, spodumene (hiddenite), euclase,
diopside, idocrase, epidote, apatite, obsidian ;
rarely diamond ; also semi-opaque, turquoise, jade.
Blue. — Corundum (sapphire), spinel, topaz, tourmaline,
zircon; among the less common species, kyanite,
iolite, benitoite, apatite; rarely diamond; also
semi-opaque, turquoise, lapis lazuli, sodalite.
Violet and Purple. — Quartz (amethyst), corundum
(oriental amethyst), spinel (almandine- spinel),
garnet (almandine), spodumene (kunzite), apatite.
Brown. — Diamond, tourmaline, quartz (smoky-quartz);
among the less common species, andalusite,
axinite, sphene.
TABLE III
Refractive Indices of Gem-Stones*
^pai .
Moonstone
'S3
* 454
'54
Iolite
'543
'SSI
Quartz .
•544
'553
Beryllonite
'553
•565
Beryl .
•578
•585
Turquoise
1-61
I-65
Topaz .
1-618
I'627
Andalusite
1-632
I '643
Tourmaline
1-626
1-651
Apatite .
1-642
1-646
Phenakite
1-652
1-667
Euclase .
1-651
1-670
Spodumene
1-660
1-675
Enstatite .
1-665
1-674
1 The least and the greatest of the refractive indices of
doubly refractive species are given.
TABLES
303
Peridot .
Axinite . . .
Diopside .
Idocrase .
Spinel .
Kyanite .
Epidote .
Garnet (Hessonite) .
Chrysoberyl .
Garnet (Pyrope)
Benitoite .
Corundum
Garnet (Almandine)
Zircon (a)
Garnet (Demantoid)
Sphene .
Zircon (b)
Diamond
1-674
1-685
1714
172
1735
1746
1757
1-761
1-901
1-927
1726
1745
1755
1790
1-815
i-88«
2-417
1-697
1-684
1705
1719
173
1766
1753
1-804
1770
I-985
1-980
Moonstone
Quartz
Beryl
Topaz
Chrysoberyl
Tourmaline
Spodumene
Corundum
Peridot
TABLE IV
Colour-Dispersion of Gem-Stones 1
. -012 Spinel .... 'O2O
. '013 Garnet (Almandine) . . '024
. -014 Garnet (Pyrope) . . '027
. '014 Garnet (Hessonite) . . -028
. "015 Zircon .... '038
. '017 Diamond .... '044
. -017 Sphene .... -051
. -018 Garnet (Demantoid) . . '057
. 'O2O
TABLE V
Character of the Refraction of Gem-Stones
(a) SINGLE—
Diamond, spinel, garnet, opal.
Diamond and garnet frequently display local double refraction.
1 The dispersion is the difference of the refractive indices correspond-
ing to the B and G lines of the solar spectrum. The value for crown-
glass is 'Oi6.
304
GEM-STONES
Quartz
Phenakite.
Apatite
Idocrase
Beryl
Chrysoberyl
Topaz
Enstatite .
Spodumene
Moonstone
lolite
Axinite
Andalusite
(6) UNIAXIAL, POSITIVE—
. -009 I Benitoite .
. '015 I Zircon (b).
Quartz exhibits circular polarization.
(c) UNIAXIAL, NEGATIVE —
•004
•005
•007
Corundum
Tourmaline
(</) BIAXIAL, POSITIVE —
. . '007 Euclase .
. '009 Diopside .
. '009 Peridot
. . "015 Sphene .
(e) BIAXIAL, NEGATIVE —
•006
•008
•oio
•on
Beryllonite
Kyanite .
Epidote .
•047
•053
•009
•025
•019
•020
•038
•084
•01?
•016
•031
TABLE VI
Dichroism of Gem-Stones
(a) STRONG
Corundum, tourmaline, alexandrite, spodumene, and-
alusite, iolite, epidote, axinite.
(£) DISTINCT
Emerald, topaz, quartz, peridot, chrysoberyl, enstatite,
euclase, idocrase, kyanite, sphene, apatite.
Beryl, diopside.
WEAK
TABLES
305
TABLE VII
Specific Gravities of Gem-Stones
Opal
. 2-15
Peridot .
. 3-40
Moonstone
. 2-57
Idocrase .
3 '4°
lolite
. 2-63 Sphene .
• 3 '4°
Quartz
. 2-66 Diamond .
VS2
Beryl
T
3 j*
Turquoise
. 2-82
Spinel
• 3'53
. 360
Beryllonite
. . 2-84
Kyanite .
• 3'6i
Phenakite .
• 2-99
Garnet (Hessonite) .
. 3'6i
Euclase .
• 3-o?
Benitoite .
• 3 '64
Tourmaline
. 3'io
Chrysoberyl
• 373
Enstatite .
• 3'i°
Garnet (Pyrope)
• 378
Andalusite
• • 3-18
Garnet (Demantoid) .
• 3^4
Spodumene
• • 3'i8
Corundum
. 4-03
Apatite .
. 3'20
Garnet (Almandine) .
• 4 '05
Axinite
. . 3-28
Zircon (a) .
. 4-20
Diopside .
. 3-29
Zircon (b) .
• 4-69
Epidote .
• 3'37j?
TABLE VIII
Degrees of Hardness of Gem-Stones
5. Kyanite (5-7), apatite, lapis lazuli
5^. Enstatite, beryllonite, sphene
6. Opal, moonstone, turquoise, diopside
G£. Spodumene, peridot, garnet (demantoid), benitoite,
idocrase, epidote, axinite, jade (nephrite)
7. lolite, quajtz, tourmaline, jade (jadeite)
7^. Garnet (hessonite, pyrope)
7^. Beryl, garnet (almandine), zircon, phenakite, euclase,
andalusite
8. Topaz, spinel
8£. Chrysoberyl
9. Corundum
10. Diamond
306
GEM-STONES
TABLE IX.— DATA
Densities of Water and Toluol at Ordinary Temperatures
TEMPERATURE
WATER
TOLUOL
Centigrade
I?
Fahrenheit
57'2°
0-9994
0-8697
15°
59 '0°
0-9992
0-8687
16
60-8°
0-9990
0-8677
17
62-6°
0-9988
0-8667
18
64-4°
0-9986
0-8657
19
66-2°
0-9985
0-8647
20
68-0°
0-9983
0-8637
21
69-0°
0-9981
0-8627
22
7r6°
0-9979
0-8617
23
73'4°
0-9977
0-8607
English carat
Metric carat
oz Av
= 0-2053 gram
= 0-2000 (one-fifth) gram
— 28*35 grams
Ib. Av.
inch
foot .
yard .
mile .
= 0*4536 kilogram
= 25*4 millimetres
= 0*3048 metre
= 0*9144 metre
= I -6093 kilometre
INDEX
Absorption, 53, 59
Baroque, Barrok, pearls, 292
Absorption spectra, 59
Bastite, 272
Achroite, 220, 221
Benitoite, 267
Adularia, 255
Berquem, Louis de, 90, 161
Agate, 247
Beryl, 184
Akbar Shah diamond, 163
Beryl lonite, 270
Alalite, 272
Bezel facet, 92
Albite, 254
Biaxial double refraction, 45, 49,
Alexandrite, 54, 60, 233
57
Scientific, 122
Bisectrix, 45, 49
Almandine, 60, 214
Black diamond, 129
Oriental, 112, 172
Black lead, 129
spinel, 112, 204
Black opal, 249, 250
Amazon-stone, 255
Black Prince's ruby, 206
Amber, 83, 298
Blister-pearl, 296
Amethyst, 239, 242
Bloodstone, 247
Oriental, III, 172, 239
Blue felspar, 255
Anatase, 281
Blue ground, 143, 147
Andalusite, 274
Blue John, 285
Andradite, 216
Boart, 103, 129, 133
Anomalous refraction, 47
Bohemian garnet (pyrope), 207,
Anorthite, 254
212
Apatite, 279
Apophyllite, 290
Bone turquoise, 259
Boodt, A. B. de, 132, 213
Aquamarine, 184, '93
Borgis, Hortensio, 161
Arizona-ruby, 213
Borneo stones, 154, 170
Artificial stones, 124
Bort, v. Boart, 103, 129, 133
Asteria, 38, 177
Bottle-stone, 284
Asterism, 38
Boule, 1 18
Australia stones, 154, 174,182,195,
Bowenite, 263
213,216,227,232, 252, 288
Braganza diamond, 170
Austrian Yellow diamond, 165
Brazil stones, 138, 165, 166, 169,
Aventurine, 240, 241
ig^etseq., 201, 215, 223,
Axes, Crystallographic, 9
236, 243, 244, 248, 266,
Optic, 49
269, 270, 274
Axinite, 278
Brazilian emerald, HI, 22O, 221
Azure-quartz, 244
peridot, 221
Azurite, 287
sapphire, ill, 221
topaz, III, 197
Balas-ruby, 203
Barnato, Barnett, 145
Brilliant form of cutting, 92
Brilliant, Scientific, 122
308
GEM-STONES
Bristol diamonds, 243
Colour dispersion, 20, 97
Bruting, 100
Conchiolin, 293
Burma stones. 178. 205, 223, 227,
Coral, 298
263
Cordierite, 266
Button-pearl, 295
Cornish diamonds, 243
Byes, By waters, 136, 150
Corundum, 172
Crocidolite, 39, 240
Cabochon form of cutting, 88
Crookes, Sir William, 132, 153
Cacholong, 251
Cross facet, 93
Cairngorm, 239
Crystal, 678
Callaica, callaina, callais, 258
Rock-, 97
Calcite, 40, 289
Cubic system, 8
California stones, 156, 195, 202,
Culet facet, 93
224, 259, 265, 267, 275
Cullinan diamond, 94, 100, 168
Californite, 264, 275
Culture pearls, 297
Cape-ruby, 213
Cumberland diamond, 164
Carat weight, 72, 84
Cyanite (Kyanite), 79, 273
Carbon, 129
Cymophane, 234
Carbonado, 129
Carborundum, 105
Darya-i-nor diamond, 162
Carbuncle, 89, 215
De Beers diamonds, 167
Carnelian, 247
Demantoid, 216
Cascalho, 139
Density, 63
Cassiterite, 281
Deviation, Minimum, 30
Cat's-eye (chrysoberyl), 38, 90,
Diamond, Characters of, 128
233
cutting, 90
(quartz), 39, 90, 240
gauges, 86
(tourmaline), 39, 219
Glaziers', 135
Hungarian, 244
Ceylon stones, 181, 195, 201, 205,
mining, 146
Occurrence of, in —
212, 215, 216, 223, 232,
Borneo, 154
236, 237, 243, 244, 255,
Brazil, 139
267, 274, 279, 284
German South-West Africa,
Ceylonese peridot (tourmaline),
ISS
221
India, 138
Ceylonite, 204
New South Wales, 154
Chalcedony, 246
Rhodesia, 155
Chatoyancy, 38
South Africa, 139
Chert, 247
Origin of, 151
Chessylite, 287
-point, 91
Chrysoberyl, 233
-rose, 92
Chrysocolla, 288
-table, 91
Chrysolite (chrysoberyl), 233
Diamonds, Classification of, 136,
(peridot), 225
149
Chrysoprase, 247
Historical, 157
Church, Sir Arthur, 6l, 211, 231
Prices of, 135
Cinnamon-stone, 211
Dichroism, >>S
Citrine, 239
Dichroite, 266
Cleavage, 80, 100, 149
Close goods, 149
Dichroscope, 55
Diffusion column, 65
Colenso diamond, 131
Diopside, 272
Colour, 53
Dioptase, 280
INDEX
309
Dispersion, Colour, 20, 24, 97
Graphite, 129
Disthene, 273
Greaser, 149
Dop, 102
Great Mogul diamond, 161
Double refraction, 28, 40
Great Southern Cross group of
Doublet, 125
pearls, 294
Dresden diamond, 171
Great Table diamond, 162
Drop-stone, 94
Great White diamond, 167
Duke of Devonshire's emerald, 191
Green garnet, 271
Greenstone, 261
Edwardes ruby, 175
Grossular, 21 1
Electrical characters, 82
Emerald, 89, 184
Habit, 12
Brazilian, 220, 221
Hardness, 78
Evening, 225
Haiiynite, 286
Oriental, III, 172
Heavy liquids, 64
Scientific, 122
Hematite, 282
Uralian, 216
Hessonite, 211
Emeraldine, 247
Hexagonal system, 10
Emery, 175
Hiddenite, 266
English Dresden diamond, 166
Hope cat's-eye, 237
Enstatite, 271
chrysolite, 237
Epidote, 275
diamond, 170
Essence d'Orient, 126
pearl, 294
Essonite (Hessonite), 21 1
sapphire, 1 21
Euclase, 269
Hornstone, 247
Eugenie diamond, 164
Hungarian cat's-eye, 244
Evening emerald, 225
Excelsior diamond, 1167
Hyacinth, 211, 228
Hydrophane, 250
Extinction, 45
Hydrostatic weighing, 72
Hypersthene, 271
Faceting machine, 105
False topaz, 239
Iceland-spar, 40, 44
Felspar, 254
Idocrase, 274
Fire, 20, 96
Imitation stones, 124
Fire-marble, 289
Imperial diamond, 167
Fire-opal, 251
Index of refraction, 16
Flats, 150
India stones, 137, 181, 194, 215,
Fleches d'amour, 240
243, 244, 248, 290
Flint, 247
Indicators, 65
Floors, 147
Indicolite, 221
Fluor, 285
Interference of light, 39, 48
Fremy, E., 115
lolite, 266
Iris, 240
Garnet, 207
Isle of Wight diamonds, 243
Green, 271
Isomorphous replacement, 13, 19
Gaudin, M. A. A., 115
Gauges, Diamond, 86
Girdle, 92
acinth, 2 1 1, 228
ade, 260
Glass, 7, 124
] adeite, 262
Gnaga Boh ruby, 180
Goniometer, 30
_ argoon, 228
] asper, 247
Grain, Pearl, 86
| ehan Ghir Shah diamond, 163
3io
GEM-STONES
Jigger, 149
Jubilee diamond, 167
Nacre, 292
Napoleon diamond, 164
Nassak diamond, 163
Kauri-gum, 299
Negative double refraction, 45
Khiraj-i-Alam ruby, 206
Nephrite, 261
Kimberlite, 152
Nicol's prism, 44
King topaz, 181, 201
Klein's solution, 67
Nizam diamond, 162
Koh-i-nor diamond, 137, 158
Obsidian, 283
Kunz, Dr. G. F., 186, 224, 262,
Occidental topaz, III, 239
265
Odontolite, 259
Kunzite, 265
Off-coloured diamonds, 130
Kyanite, 79, 273
Olivine (demantoid), 216
(peridot), 225
Labradorite, 255
Onyx, 247
La Pellegrina pearl, 294
Opal, 39, 249
Lapis lazuli, 286
Fire, 251
Lazurite, 286
-matrix, 251
Lozenge facet, 93
Opalescence, 39
Lumachelle, 289
Optical anomalies, 47
Lustre, 37
Optic axes, 49
Oriental almandine, 112, 172
Maacles, Macles, 12, 150
amethyst, III, 172
Madagascar stones, 195, 224, 243,
emerald, in, 172
265, 266
topaz, III, 172
Malachite, 287
Malacolite, 272
Orient of pearls, 292
Orloff diamond, 160
Manufactured stones, 113
Ortboclase, 254
Marble, 289
Orthorhombic system, 1 1
Mattan diamond, 155, 170
Matura diamonds, 232
Pacha of Egypt diamond, 1 65
Mazarin, Cardinal, 92
Paste, 47, 124
Meerschaum, 288
Paul I diamond, 171
Melee, 136
Pavilion, 93
Methylene iodide, 26, 66
Pavilion facet, 93
Metric carat, 85, 87
Pear-drop pearls, 292
Milky-quartz, 240
Minimum deviation, 30
Pear-eye pearls, 292
Pearl, 291
Mocha-stone, 247
grain, 86
Moe's gauge, 87
Mohs's scale of hardness, 78
imitations, 126
Pendeloque, 94
Moissan, Henri, 153
Peridot, 225
Moldavite, 283
Brazilian, 221
Monoclinic system, 1 1
Ceylonese, 221
Moon of the Mountains diamond,
Peruzzi, Vincenzio, 92
162
Phenakite, 269
Moonstone, 39, 255
Pigott diamond, 164
Morganite, 186, 195
Pipes, 152
Moroxite, 279
Moss-agate, 247
Pistacite, 275
Pitt diamond, 100, 159
Mother-of-emerald, 240
Plasma, 247, 264
Mother-o'-pearl, 292 Pleochroism, 57
INDEX
Pleonaste, 204
Schorl, 221
Pliny, 6, 88, 138, 184, 191, 241,
Scientific alexandrite, 122
249
brilliant, 122
Polar Star diamond, 163
emerald, 122
Polarization, 42
topaz, 121
Porter-Rhodes diamond, 166
Scotch topaz, 239
Positive double refraction, 45
Seed pearls, 294
Prase, 240, 247
Serpentine, 289
Prehnite, 278
Setting of gem-stones, 107
Pycnometer, 75
Shah diamond, 163
Pyrites, 282
Sheen, 39
Pyrope, 212
Shepherd's Stone diamond, 163
Siam stones, 180
Quartz, 50, 238
Siberia and Asiatic Russia stones,
Quoin facet, 93
182, 188, 194, 201, 217,
223, 236, 244, 256, 262,
Rainbow-quartz, 240
269, 270, 287
Reconstructed stones, 116
Siberite, 221
Reef, 144
Siderite, 244
Reflection of light, 14
Silver-thallium nitrate, 69
Refraction of light, 15
Skew facet, 93
Refractive index, 16
Skill facet, 93
Refractometer, 22, 50
Smoky quartz, 240
Regent diamond, 100, 159
Snell's laws, 16
Retgers's salt, 69
Soapstone, 288
Rhodes, Cecil]., 145
Sodalite, 286, 287
Rhodesia stones, 155, 183, 213, 236
Sonstadt's solution, 67
Rhodolite, 62, 214
South Africa stones, 139 et seq.,
Rhodonite, 287
166, 167 et seq., 213, 232,
Rock-crystal, 97, 239
Rock-drill, 134
244, 264, 271
Spanish topaz, 239
Rontgen rays, 83
Rose form of cutting, 91
Specific gravity, 63
Specific-gravity bottle, 75
Rose-quartz, 240
Rospoli sapphire, 182
Rotation of plane of polarization,
Spectroscope, 59
Spectrum, 20, 25
Spectium, Absorption, 59
50
Spessartite, 216
Rubellite, 220, 223
Sphene, 276
Rubicelle, 203
Spinel, 203
Ruby, 98, 110, 172
Spodumene, 265
Balas-, 203
Spotted stones, 149
Cape-, 213
Star-facet, 92
Star of Africa diamond, 168
Sancy diamond, 161
Star of Este diamond, 165
Sapphire, 98, no, 172
Star of Minas diamond, 169
Brazilian (tourmaline), 221
Star of South Africa diamond,
-quartz, 244
141, 166
Water- (iolite), 266
Star of the South diamond, 139,
Water- (topaz), 201
165
Sard, 247
Starstones, 38, 177
Sardonyx, 247
Steatite, 288
Saussurite, 263
Step form of cutting, 98
312
GEM-STONES
Stewart diamond, 166 Turquoise-matrix, 2&
btrass, 124
Sunstone, 255
Synthetical stones, 113
Tuscany diamond, 165
Twinning, 12, 47
Syriam, Syrian, garnet, 215
Uniaxial double refraction, 45, 48
Table facet, 92
Table form of cutting, 91
Tavernier, J. B., 91, 129, 137, 161,
Uralian emerald, 217
Uvarovite, 218
162, 170
Templet facet, 92
Variscite, 259
Tetragonal system, 9
Verdite, 264
Thulite, 289
Verneuil, A. V. L., 116
Tiffany diamond, 171
Vesuvianite, 274
Tiger's-eye, 39, 240
Victoria diamond, 167
Timur-ruby, 206
Violane, 287
Titanite, 276
Topaz, 197
Brazilian, 197
False, 239
Occidental, ill, 239
Oriental, in, 173
Scientific, 121
Scotch, 239
Spanish, 239
Topazolite, 216
Total-reflection, 18, 21
Wart-pearl, 296
Water (of diamonds), 129
(of pearls), 292
Water-chrysolite, 284
-sapphire (iolite), 266
-sapphire (topaz), 201
White opal, 249
White Saxon diamond, 165
Wollaston.W. H., 133
Tourmaline, 43, 219
Trap form of cutting, 98
X-rays, 83
Trichroism, 57
Triclinic system, 12
Triplet, 126
Yellow ground, 143
Turquoise, 257
Zircon, 228
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MAYOR OF TROY, THE. "Q."
Muss DECK, THE. W. F. Shannon.
MIGHTY ATOM, THB. Marie Corelli.
MIRAGE. E. Temple Thurston.
MISSING DELORA, THE. E. Phillips Oppcn-
heim.
MR. GREX OF MONTH CARLO. E. Phillips
Oppenheim.
MR. WASHINGTON. Marjorie Bowen.
MRS. MAXON PROTESTS. Anthony Hope.
MRS. PETER HOWARD. Mary E. Mann.
MY DANISH SWEETHEART. W. Clark
Russell.
MY FRIEND THE CHAUFFTCUR. C. N. and
A. M. Williamson.
MY HUSBAND AND I. Leo Tolstoy.
MY LADY OF SHADOWS. John Oxenham.
MYSTERY OF DR. FU-MANCHU, THE. Sax
Rohmer.
MYSTERY OF THE GREEN HEART, THE.
Max Pemberton.
NINE DAYS' WONDEB, A. B. M. Croker.
NINK TO Srx-THiSTT. W. Pett Ridge.
OCEAN SLEUTH, THE. Maurice Drake.
OLD ROSE AND SILVER. Myrtle Reed.
PATHS OF THE PRUDENT, THE. J. S. Fletcher.
PATHWAY OF THE PIONEER, THE. Doll
Wyllarde.
PEGGY OF THB BARTONS. B. M. Croker.
PEOPLE'S MAN, A. E. Phillips Oppenheim,
PETER AND JANE. S. Macnaughtan.
POMP OF THE LAVILETTES, THE. Sir Gilbert
Parker.
QUEST OF GLORY, THB. Marjorie Bowen.
QUEST OF THB GOLDEN ROSE, THE. John
Oxenbam. ,
REGENT, THE. Arnold Bennett.
REMINGTON SENTENCE, THB. W. Pett
Ridge.
REST CURE, THB. W. B. MaxwelL
RETURN OF TARZAN, THE. Edgar Rice
Burroughs.
ROUND THE RED LAMP. Sir A. Conan Doyle.
ROYAL GEORGIB. S. Baring-Gould.
SAID, THB FISHERMAN. Marmaduke Pick-
thall.
SALLY. Dorothea Conyers,
SALVING OF A DERELICT, THE. Maurice
Drake.
SANDY MARRIED. Dorothea Conyers.
SEA CAPTAIN, THE. H. C. Bailey.
SEA LADY, THE. H. G. Wells.
SEARCH PARTY. THE. George A. Birmingham.
SECRET AGENT, THE. Joseph Conrad.
SECRET HISTORY. C. N. and A. M. William-
son.
SECRET WOMAN, THE. Eden Phillpotts.
SET IN SILVER. C. N. and A. M. William-
son.
SEVASTOPOL, AND OTHER STORIES, Leo
Tolstoy.
SEVERINS, THE. Mrs. Alfred Sidgwick.
SHORT CRUISES. W. W. Jacobs.
SI-FAN MYSTERIES, THE. Sax Rohmer.
SPANISH GOLD. George A. Birmingham.
SPINNER IN THE SUN, A. Myrtle Reed.
STREET CALLED STRAIGHT, THE. Basil
King.
SUPREME CRIME, THE. Dorothea Gerard.
TALES OF MEAN STREETS. Arthur Morrison.
TARZAN OF THIS APKS. Edgar Ric« Btx-
FlCTIOW
Hethuon** Cheap Xov tin— continued.
TERESA OF WATLING STREET. Arnold
Bennett.
THERE WAS A CROOKED MAN. Dolf Wyllarde.
TYRANT, THE. Mrs. Henry de la Pasture.
UNDER WESTERN EVES. Joseph Conrad.
UNOFFICIAL HONEYMOON, THE. Dolf
Wyllarde.
VALLEY OF THB SHADOW, THB. William
Le Queux.
VIRGINIA PERFECT. Peggy Webling.
WALLET OF KAI LUNG. Ernest Bramah.
WAR WEDDING, THB. C. N. and A. M.
Williamson.
WARE CASE, THE. George PleydelL
WAY HOMK, THE. Basil King.
WAT OF THESE WOMEN, THK. E. Phillips
Oppenheim.
WEAVER OF DREAMS, A. Myrtle Reed.
WEAVER OF WEBS, A. John Oxenham.
WEDDING DAY, THE. C. N. and A. M.
Williamson.
WHITE FANG. Jack London.
WILD OLIVE, THB. Basil King.
WILLIAM, BY THE GRACE OF GOD. Marjorie
Bowen.
WOMAN WITH THB FAN, THE. Robert
Hichens.
WO* Maurice Drake.
WONDER OF LOVE, THE. E. Maria Albanesi.
YELLOW CLAW, THE. Sax Rohmer.
YELLOW DIAMOND, THE. Adeline Sergeant.
Methuen's One and Threepenny Novels
BARBARA RESELL. Mrs. Eelloc Lowndes.
Bv STROKE OF SWORD. Andrew Balfour.
DERRICK VAUGHAN, NOVELIST. Edna
LyalL
HOUSE OF WHISPERS, THK. William Le
Queux.
INCA'S TREASURE, THE E. Glanville.
KATHERINK THB ARROGANT. Mrs. B. M.
Croker.
MOTHER'S SON, A. B. and C. B. Fry.
PROFIT AND Loss. John Oxenham.
RED DERELICT, THB. Bertram Mitford.
SIGN OF THK SPIDER, THB. Bertram Mitford.
37/6/19.
PRINTSP BY MORRISON AND GIBB LIMITED, EDINBURGH
UC SOUTHERN REGIONAL
A 000 020 479 2
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